,RTH 

:NCES 

RARY 


pale  'Bicentennial 

STUDIES   IN    EVOLUTION 


gale  'Bicentennial  publication?! 

With  the  approval  of  the  President  and  Fellows 
of  1C  ale  University,  a  series  of  volumes  has  been 
prepared  by  a  number  of  the  Professors  and  In- 
structors, to  be  issued  in  connection  with  the 
Bicentennial  Anniversary,  as  a  partial  indica- 
tion of  the  character  of  the  studies  in  which  the 

University  teachers  are  engaged. 

/ 

This   series   of  volumes    is    respectfully  dedicated  to 

of  tlje 


STUDIES  IN  EVOLUTION 


MAINLY  REPRINTS  OF  OCCASIONAL JPAPERS^  ' 
SELECTED  FROM  THE 

PUBLICATIONS  OF  THE  LABORATORY  OF  INVERTEBRATE 

PALEONTOLOGY,    PEABODY  MUSEUM 

YALE   UNIVERSITY 


BY 

CHARLES  EMERSON  BEECHER 


NEW   YORK:    CHARLES   SCRIBNER'S   SONS 

LONDON:   EDWARD   ARNOLD 

1901 


EARTH 
CIENC 
UBRARY 


Copyright,  1901, 
BY  YALE    UNIVERSITY 

Published^  August,  IQOI 


EARTH 


UNIVERSITY  PRESS    •    JOHN    WILSON 
AND    SON    •     CAMBRIDGE,    U.S.A. 


PREFACE 

THE  following  papers  from  the  publications  of  the  Labora- 
tory of  Paleontology  have  been  selected  for  reprinting  on 
account  of  their  representing  more  or  less  closely  a  distinct 
line  of  research;  namely,  the  investigation  and  study  of  the 
development  of  a  number  of  invertebrate  animals.  The  gen- 
eralizations resulting  from  such  studies  properly  belong  to 
the  province  of  organic  evolution,  while  the  detailed  methods 
pertain  to  the  observation  and  interpretation  of  the  stages  of 
growth  and  decline  in  the  organism. 

Nearly  all  the  subject-matter  i^  based  upon  studies  of  the 
remains  of  fossil  animals,  many  of  them  coming  from  the 
oldest  known  fossil-bearing  rocks.  In  some  instances  material 
representing  living  species  has  been  introduced  for  comparison 
and  to  illustrate  further  the  problems  under  investigation. 

The  first  work  done  in  America  on  the  stages  of  growth 
of  fossil  Brachiopoda  was  a  memoir  by  the  present  writer,  in 
collaboration  with  Dr.  John  M.  Clarke,  on  "  The  Development 
of  Some  Silurian  Brachiopoda,"  published  by  the  University 
of  the  State  of  New  York.  It  seems  fitting  to  include  this, 
in  order  to  complete  the  work  on  the  development  of  the 
Brachiopoda  carried  on  subsequently  by  the  writer.  Another 
joint  paper,  written  in  connection  with  Mr.  Charles  Schuchert, 
is  also  introduced. 

The  first  paper  in  the  present  collection,  on  "  The  Origin  and 
Significance  of  Spines,"  is  an  attempt  to  apply  the  general  laws 
of  evolution  in  the  study  of  a  particular  structure  throughout 

938005 


Vlll 


PREFACE 


the  animal  and  vegetable  kingdoms,  and  to  discover  its  true  in- 
terpretation in  terms  of  ontogeny,  phylogeny,  and  chronology. 

The  next  division  of  the  volume  comprises  papers  on  the 
structure  and  development  of  Trilobita.  The  following  section 
presents  developmental  studies  on  the  Brachiopoda.  It  should 
be  stated  that  this  work  was  undertaken  largely  in  the  hope 
that  the  results  would  lead  to  the  principles  governing  a  natural 
classification  of  all  forms  in  these  two  classes.  In  the  brachio- 
pods  nothing  further  than  a  division  into  orders  and  a  grouping 
of  the  families  under  the  orders  was  attempted.  The  elabora- 
tion of  this  classification  has  been  very  fully  carried  out  by 
Mr.  Charles  Schuchert,  in  his  "  Synopsis  of  American  Brachiop- 
oda." For  the  Trilobita,  a  new  arrangement  into  orders  was 
suggested,  together  with  a  redefining  of  the  families  and  a 
grouping  of  the  genera  under  them. 

In  the  last  division  are  three  papers  on  special  problems  of 
development. 

The  author  is  greatly  indebted  to  Miss  Lucy  P.  Bush  for 
assistance  in  arranging  the  material  for  this  volume,  and 
especially  for  aid  while  it  was  being  printed. 

YALE  UNIVERSITY,  April  3,  1901. 


CONTENTS 


PAGE 

I.   GENERAL  EVOLUTION 

1.   THE  ORIGIN  AND  SIGNIFICANCE  OF  SPINES 3 

Introduction 3 

Law  of  Variation 5 

Definition  of  Terms 9 

Growth  of  a  Spine 10 

Localized  Stages  of  Growth 14 

Compound  Spines 16 

Application  of  Law  of  Morphogenesis 17 

Ontogeny  of  a  Spinose  Individual 18 

Phylogeny  of  Spinous  Forms 23 

Categories  of  Origin 26 

Conditions  or  Forces  affedting  Growth 31 

A.  External  Stimuli 32 

B.  Growth  Force     .     .     ...    >     .     ....  34 

C.  External  Restraint 38 

D.  Deficiency  of  Growth  Force 39 

Summary  of  Causes  of  Spine  Genesis 41 

I.    In  response  to  stimuli  from  the  environment  act- 
ing on  most  exposed  parts.     (Aj.)  ....  42 
II.    As  extreme  results  of  progressive  differentiation 

of  previous  structures.    (A2,  B3.)     .     .     .     .  47 

III.  Secondarily  as  a  means  of  protection  and  offence. 

(AftB4.) 52 

IV.  Secondarily  from  sexual  selection.    (A4,  B4.)       .  57 
V.    Secondarily  from  mimetic  influences.     (A6,  B4.)  .  60 

VI.    Prolonged  development  under  conditions  favor- 
able for  multiplication.     (B!.) 64 

VII.    By  repetition.    (B2.) 67 

VIII.    Restraint  of  environment  causing  suppression  of 

structures.    (Cx.) 70 

IX.   Mechanical  restraint.    (C2.) 77 

X.    Disuse.     (C3,  D2.) 80 

XI.    Intrinsic  suppression  of  structures  and  functions. 

D. 86 


x  CONTENTS 

PAGE 

1.  THE  ORIGIN  AND    SIGNIFICANCE    OF    SPINES  —  Con- 
tinued 

Categories  of  Interpretation 93 

Spinosity  a  Limit  to  Variation 93 

Spinosity  the  Paracme  of  Vitality 97 

Conclusion 99 

References 102 

II.   STRUCTURE  AND  DEVELOPMENT  OF  TRILOB1TES 
1.   OUTLINE    OF    A    NATURAL    CLASSIFICATION    OF    THE 

TRILOBITES 109 

Introduction 109 

Previous  Classifications 110 

Rank  of  the  Trilobites 114 

Comparative  Morphology  of  Crustacea 115 

Morphology  of  the  Cephalon ...117 

Principles  of  a  Natural  Classification 119 

Application  of  Principles  for  Ordinal  Divisions    .     .     .  121 
Application  of  Principles  for  Arrangement  of  Families 

and  Genera 125 

Diagnoses  and  Discussions 130 

Arrangement  of  the  Families  of  Trilobites 131 

Diagnoses  and  Discussions  of  Orders  and  Families  .     .  134 

Hypoparia 134 

Family  I.        Agnostidas     . 135 

Family  II.      Harpedidae 137 

Family  III.    Trinucleidaa 138 

Opisthoparia 138 

Family  IV.     Conocoryphidae 140 

Family  V.      Olenidas 141 

I.  Paradoxinse 143 

II.  Oryctocephalinae 145 

III.  Oleninae 145 

IV.  Dikelocephalinae 145 

Family  VI.     Asaphidse 145 

I.  Asaphidas 146 

II.  Illsenidae ' 146 

Family  VII.       Proetidse         147 

Family  VIII.     Bronteidae 149 

Family  IX.         Lichadidse 150 

Family  X.          Acidaspidas 151 

Proparia 152 

Family  XI.       Encrinuridse 153 

Family  XII.     Calymmenidae 154 


CONTENTS  xi 

PAGE 

1.  OUTLINE    OF  A   NATURAL    CLASSIFICATION    OF    THE 

TRILOBITES  —  Continued 

Family  XIII.     Cheiruridse 155 

Family  XIV.     Phacopidse      .......  15€ 

References 157 

List  of  Genera 159 

2.  THE  SYSTEMATIC  POSITION  OF  THE  TRILOBITES      .     .  163 
8.   THE  LARVAL  STAGES  OF  TRILOBITES 166 

Introduction 166 

The  Protaspis 167 

Review  of  Larval  Stages  of  Trilobites 171 

Analysis  of  Variations  in  Trilobite  Larvse 179 

Antiquity  of  the  Trilobites 183 

Restoration  of  the  Protaspis 185 

The  Crustacean  Nauplius 188 

Summary 193 

References 195 

4.  ON  THE  MODE  OF  OCCURRENCE,  AND  THE  STRUCTURE 

AND  DEVELOPMENT  OF  TRIARTHRUS  BE  OKI    .     .  197 

5.  FURTHER   OBSERVATIONS    ON    THE   VENTRAL    STRUC- 

TURE OF  TRIARTHRUS 203 

Paired  Uniramose  Appendages 205 

Anterior  Antennae,  or  Antennules 205 

Paired  Biramous  Appendages 205 

First  Pair  of  Biramous  Appendages,  or  Posterior 

Antennae 206 

Second  Pair  of  Biramous  Appendages,  or  Mandibles  206 

Third  and  Fourth  Biramous  Appendages,  or  Maxillae  206 

Thoracic  Legs 207 

Organs  in  the  Median  Line 208 

The  Hypostoma 208 

The  Mouth 209 

The  Metastoma 209 

The  Anal  Opening 209 

Observations 210 

Summary  of  Ventral  Organs  of  Triarthrus      .     .     .     .  211 

6.  THE  MORPHOLOGY  OF  TRIARTHRUS 213 

References 219 

7.  STRUCTURE  AND  APPENDAGES  OF  TRINUCLEUS    .     .     .  220 

Appendages 223 

Endopodites 224 

Exopodites 224 


xii  CONTENTS 

PAGE 

III.   STUDIES  IN  THE  DEVELOPMENT  OF  THE  BRACH- 
IOPODA 

1.  DEVELOPMENT  OF  THE  BRACHIOPODA 229 

I.  Introduction 229 

The  Protegulum 230 

Affinities    . 232 

Modifications  from  Acceleration 233 

Differences  in  the  Valves 234 

Genesis  of  Form 238 

Types  of  Pedicle-openings      . 240 

Atremata 243 

Neotremata     .                          244 

Protremata 244 

Telotremata 245 

II.  Classification  of  the  Stages  of  Growth  and  Decline  .  246 

Embryonic  Stages 247 

Larval  Stages 250 

Origin  of  the  Deltidium  and  Deltidial  Plates .     .  257 

Post-embryonic  Stages 265 

Nepionic  Period 267 

Neanic  Period 268 

Ephebic  Period 269 

Gerontic  Period 269 

Synopsis 271 

References 272 

III.  Morphology  of  the  Brachia 274 

Classification  of  Brachial  Structures  ....  276 

Leiolophus  Stage 277 

Taxolophus  Stage 277 

Trocholophus  Stage 278 

Schizolophus  Stage 278 

Ptycholophus  Stage 280 

Zugolophus  and  Plectolophus  Stages  .  .  .  281 

Spirolophus  Stage 282 

References 285 

2.  SOME  CORRELATIONS  OF  ONTOGENY  AND  PHYLOGENY 

IN  THE  BRACHIOPODA 286 

3.  REVISION  OF  THE  FAMILIES  OF  LOOP-BEARING  BRACH- 

IOPODA    290 

The  Terebratulidae 290 

The  Terebratellidae 291 

Magellaniinae 293 

Dallininae 295 

Comparisons  and  Homologies 299 

Morphogeny  from  Gwynia  to  Megathyris 302 


CONTENTS  xiii 

PAGE 

3.  REVISION  OF  THE  FAMILIES  OF  LOOP-BEARING  BRACHI- 
OPODA  —  Continued 

Morphogeny  from  Gwynia  to  Dallina 303 

Morphogeny  from  Gwynia  to  Magellania 303 

Conclusions 304 

Classification 305 

Family  Terebratulidse  Gray 306 

Centronellinae  Waagen 306 

Stringocephalinae  Dall 307 

Terebratulinae  Dall 307 

Dyscoliinae  (=Dyscoliid8e  Fischer  and  (Ehlert 

emend.) 307 

Family  Terebratellidae  King  emend 307 

Dallininae  n.  sub.-fam 307 

Magellaniinse  n.  sub.-fam 308 

Megathyrinse  Dall 308 

References 308 

4.   DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA    .     .  310 

Introduction 310 

List  of  the  Brachiopoda  occurring  in  the  Niagara 

Shales  at  Waldron,  Indiana 314 

Discussions  of  the  Species 317 

Crania  siluriana  Hall,,  1863 317 

Dalmanella  elegantula  Dalman,  1827 317 

Specific  Characters 318 

Mature  Form 318 

Incipient  Form 319 

Developmental  Changes 320 

General  Form  and  Outline 320 

Beaks 320 

Foramen 320 

Plications 321 

RJiipidomella  hybrida  Sowerby,  1839 321 

Leptcena  rhomboidalis  Wilckens,  1769 322 

Specific  Characters 323 

Mature  Form 323 

Incipient  Form 323 

Developmental  Changes 324 

Development  of  Leptcena  rhomboidalis  .     .     .  325 

Orthothetes  subplanus  Conrad,  1842 327 

Specific  Characters 328 

Mature  Form 328 

Incipient  Form 328 

Developmental  Variations 329 

Strophonella  striata  Hall,  1843 330 

Specific  Characters 331 


xiv  CONTENTS 

PAGE 

4.  DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA  —  Con- 
tinued 

Mature  Form 331 

Incipient  Shell 331 

Developmental  Changes 331 

Mimulus  waldronensis  Miller  and  Dyer,  1878       .     .  334 

Dictyonella  reticulata  Hall,  1868 335 

Anastrophia  internascens  Hall,  1879 337 

Specific  Characters 337 

Mature  Form 337 

Incipient  Form 338 

Developmental  Changes 338 

Camarotcechia  acinus  Hall,  1863 339 

Specific  Characters 340 

Mature  Form 340 

Variations  from  the  Normal  Adult  ....  340 

Initial  Shell 340 

General  Developmental  Characters 341 

Camarotcechia  neglecta  Hall,  1852 341 

Specific  Characters 342 

Mature  Form 342 

Incipient  Form 343 

Developmental  Variations 343 

General  Form  and  Outline 343 

Beak  and  Foramen 343 

Plications 343 

Camarotcechia  Whitii  Hall,  1863 344 

Specific  Characters 344 

Mature  Form 344 

Abnormalities  at  Maturity 345 

Incipient  Form 345 

Developmental  Variations 345 

General  Form  and  Outline 345 

Beak  and  Foramen 346 

Plications 346 

Camarotcechia  indianensis  Hall,  1863 346 

Specific  Characters 347 

Normal  Mature  Form 347 

Variations  from  the  Normal 348 

A.  Forms  with  one  plication  in  the  ven- 

tral sinus 348 

B.  Forms  with  three  plications  in   the 

ventral  sinus .     . 348 

C.  Forms  with  four  plications  in  the  ven- 

tral sinus 348 

Monstrous  Forms 348 

Initial  Shell  349 


CONTENTS  xv 

PAGE 

4.   DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA  —  Con- 
tinued 

Developmental  Variations 349 

General  Form  and  Outline 349 

Beak 350 

Foramen 350 

Plications 351 

Rhynchotreta  cuneata  Dalinan,  1827,  var.  americana 

Hall,  1879 351 

Specific  Characters 352 

Mature  Form 352 

Incipient  Form 353 

Developmental  Changes 353 

Contour 353 

Fold  and  Sinus 353 

Beak 354 

Surface  Ornaments 354 

Cardinal  Area 355 

Variations 356 

Atrypa  reticularis  Linnaeus,  1767 356 

Specific  Characters 356 

Mature  Form 356 

Incipient  Form   .'.........  357 

Developmental  Variations 358 

General  Form  and  Outline 358 

Beak 358 

Foramen ,     .     .     .  358 

Plications 359 

Summary 359 

Homceospira  evax  Hall,  1863 360 

Specific  Characters 360 

Mature  Form 360 

Variations  from  the  Normal  Development      .  361 

Developmental  Variations 362 

Beaks 363 

Foramen 363 

Sinus 364 

Plications 364 

Internal  Apparatus 364 

Homceospira  sobrina  sp.  nov 366 

Specific  Characters 367 

Mature  Form 367 

Variations  from  the  Normal  Mature  Form    .  368 

Incipient  Form 368 

Developmental  Variations 368 


CONTENTS 

PAGE 

4.  DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA — Con- 
tinued 

General  Form  and  Outline 368 

Beak  and  Foramen 369 

Atrypina  disparilis  Hall,  1852 369 

Specific  Characters 370 

Mature  Form 370 

Variations  in  Outline 370 

Abnormalities 370 

Developmental  Changes 371 

Meristina  rectirostris  Hall,  1882 372 

Specific  Characters 373 

Mature  Form 373 

Incipient  Form 373 

Developmental  Variations 374 

General  Form  and  Outline      ......  374 

Beak 374 

Foramen 374 

Whitfieldella  nitida  Hall,  1843 374 

Specific  Characters 375 

Mature  Form 375 

Variations  in  Outline 375 

Incipient  Form 376 

Developmental  Variations 376 

Meristina  Maria  Hall,  1863 377 

Specific  Characters 378 

Mature  Form 378 

Incipient  Form 379 

Developmental  Variations 379 

General  Form  and  Outline 379 

Beak 379 

Foramen 379 

Spirifer  crispus  Hisinger,  1826 380 

Spirifer  crispus,  var.  simplex  Hall,  1879      ....  380 
Reticularia  bicostata  Vanuxem,  1842,  var.  petila  Hall, 

1879 380 

Spirifer  radiatus  Sowerby,  1825 382 

Incipient  Form 382 

Developmental  Changes 383 

Summary  of  Developmental  Changes 386 

Size  and  Contour 386 

Valves 388 

Beaks 389 

Cardinal  Area 389 

Internal  Apparatus 395 

Surface  Ornaments  .........  396 

Varieties  and  Abnormalities  .  397 


CONTENTS  xvii 

PAGE 

5.  DEVELOPMENT  OF  BILOBITES 399 

Developmental  Changes  in  BUobites  various     ....  402 

Observations 404 

6.  DEVELOPMENT  OF  TEREBRATALIA  OBSOLETA  DALL    .     . 

7.  DEVELOPMENT   OF   THE  BRACHIAL   SUPPORTS   IN  DIE- 

LASMA   AND   ZYGOSPIRA 410 

Development  of  the  Loop  in  Dielasma  turgidum   .     .     .  412 
Development  of  the  Brachial    Supports   in   Zygospira 

recurvirostris 413 

Observations  and  Correlations 415 

IV.   MISCELLANEOUS   STUDIES  IN  DEVELOPMENT 

1.  DEVELOPMENT  OF  A  PALEOZOIC  PORIFEROUS  CORAL  .     .  421 

Development  of  Pleurodictyum  lenticular e 422 

General  Conclusions 425 

2.  SYMMETRICAL  CELL  DEVELOPMENT  IN  THE  FAVOSITID^E  429 

Summary 433 

3.  DEVELOPMENT   OF    THE    SHELL   IN   THE   GENUS    TOR- 

NOCERAS  HYATT 435 

V.   PLATES  AND  EXPLANATIONS.  441 


INDEX 597 


ILLUSTRATIONS 


LIST   OF  PLATES 

I.  Spines  of  Radiolaria. 

II.  Classification  of  Trilobites. 

III.  Larval  Stages  of  Trilobites. 

IV.  Larval  Stages  of  Trilobites. 
V.  Crustacean  Larvae. 

VI.  Appendages  of  Triarthrus. 

VII.  Ventral  Side  of  Triarthrus. 

VIII.  Appendages  of  Triarthrus. 

IX.  Triarthrus. 

X.  Appendages  of  Trinucleus. 

XI.  Stages  of  Growth  of  Brachiopoda. 

XII.  Stages  of  Growth  of  Brachiopoda. 

XIII.  Parallelism  in  Brachiopoda  (Magellania  Series). 

XIV.  Ontogeny  and  Phylogeny  of  the  Terebratellidae. 
XV.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XVI.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XVII.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XVIII.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XIX.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XX.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XXI.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XXII.  Stages  of  Growth  in  Silurian  Brachiopoda. 

XXIII.  Development  of  BiloUtes. 

XXIV.  Development  of  Terebratalia. 
XXV.  Development  of  Terebratalia. 

XXVI.  Brachial  Supports  in  Dielasma  and  Zygosplra. 

XXVII.  Development  of  Pleurodictyum. 

XXVIII.  Pleurodictyum. 

XXIX.  Pleurodictyum. 

XXX.  Pleurodictyum. 

XXXI.  Pleurodictyum  and  Favosites. 

XXXII.  Favositidaj. 

XXXIII.  Favositidse. 

XXXIV.  Tornoceras. 


xx  ILL  US  TRA  TIONS 


FIGURES   IN  TEXT 

FIGURE  PAGE 

1-5.  Different  stages  of  growth  of  a  spine 10 

6.  A  profile  of  a  single  radiating  ridge  of  Spondylus  princeps ; 

showing  the  series  of  flattened  spines 10 

7-12.  Diagrams  ;  showing  growth  and  differentiation  of  ornament 

into  spines . 11 

13.  Summer  shoot  of  Barberry  ;   showing  the  gradations  between 

leaves  and  spines 12 

14.  Profile  of  one  of  the  primary  rays  of  Spondylus  imperialis  ;  show- 

ing the  series  of  spines 12 

15.  Example  of  spine  growth  of  simple  increscence 13 

16.  Stages  of  spine  growth  by  successive  replacement     ....     .  13 

17.  Stages  of  spine  growth  by  serial  repetition 13 

18.  Stages  of  spine  growth  by  decrescence 13 

19.  Sector;  showing  in  diagram  the  multiplication  of  radiating  lines 

by  interpolation 15 

20.  Profiles  of   spines  produced  on  the  various    radii  at  the  four 

zones;  as  indicated  in  the  preceding  figure 15 

21.  Simple  spine 16 

22.  Spine,  with  lateral  spinules 16 

23.  Spine,  with  forked  apex  and  lateral  spinulose  spinules       ...  16 

24.  Prodissoconch  of  Ostrea  virginiana    . 20 

25.  Each  stage  of  Avicula  sterna 20 

26.  Young  of  Avicula  sterna ;  showing  the  beginning  of  spine  growth  20 

27.  Young  Saxicava  arctica 20 

28.  Young  Anomia  aculeata 20 

29.  Young  Spondylus  princeps 20 

30.  Side  view  of  Spondylus  calcifer ;  about  one-third  grown  ;  show- 

ing the  characteristic  spinous  growth 21 

31.  Side  view  of  Spondylus  calcifer  ;  showing  the  greatly  thickened 

right  valve  and  the  entire  absence  of  spines  over  the  whole 

shell 21 

32.  Attlieya  decora,  a  diatom,  with  spines  from  the  angles   ....  44 

33.  Difflugia  acuminata,  a  freshwater  rhizopod;  showing  spiniform 

projection  of  the  fundus 44 

34.  Difflugia  constricta,  a  freshwater  rhizopod,  with  rounded  fundus  44 

35.  The  same;  showing  a  single  spine  on  the  fundus 44 

36.  The  same;  showing  two  spines 44 

37.  Cyaihophycus  reticulatus.     Ordovician 49 

38.  Dictyospongia  Conradi.     Devonian 49 

39.  Hydroceras  tuberosum.     Devonian 49 


ILL  USTRA  T10NS  xxi 

FIGURE  PAGE 

40.  Lima  squamosus 51 

41.  Antler  of  Cervulus  (?)  dicranoceras 53 

42.  Antler  of  Cervus  pardinensis -— JJ3L 

43.  Antler  of  the  Fallow  Deer  (Cervus  dama) 53 

44.  Zoe'a  of  the  common  crab  (Cancer  irroratus) 56 

45.  Profile  of  head  of  Chamceleon  Oweni ;  male 60 

46.  Female  of  the  same  species 60 

47.  Profile  of  a  spider  (Ccerostris  mitralis)  on  a  twig  mimicking  a 

spiny  excrescence 61 

48.  The  larva  of  the  Early  Thorn  Moth  (Selenia  illunarid)  resting 

on  a  twig  ;  showing  mimicry  of  stem  and  spiniform  processes       61 

49.  Australian  Pipe-fish  (Phyllopteryx  eques)  and  frond  of  sea-weed 

in  lower  right-hand  corner;  showing  mimicry 62 

50.  Allorchestes    armatus,  a  spiny  amphipod  from    Lake  Titicaca; 

female 65 

51.  Acontaspis    hastata,  a    radiolarian ;    showing   multiplication   of 

spines  by  repetition 69 

52.  Strophalosia  keokuk,  an  attached  brachiopod;  showing  the  spines 

extending  from  the  ventral  valve  to  and  along  the  surface  of 
attachment 69 

53.  A  gastropod  shell  (Platyceras)  to  which  are  attached  a  number 

of  Strophalosia  keokuk 69 

54.  The   spiny     Cytlsus    (C.     spinosus);    showing    suppression    of 

branches  into  spines 75 

55.  A   single   leaf   of   Tragacanth   (Astragalus    Tragacantha),   from 

which  the  three  upper  leaflets  have  fallen 75 

56.  Leaf  axis  of  the  same,  from  which  all  the  leaflets  have  fallen  .     .       75 

57.  Twig  of  common  locust  (Robinia  Pseudacacia)  ;  showing  spines 

representing  stipules 75 

58.  Portion  of  skin  of  Python  ;  showing  the  spurs  which  represent 

the  suppressed  or  vestigial  hind  legs 76 

59.  Bones  of  suppressed  legs  of  Python 76 

60.  Dorsal  view  of  Spirifer  mucronatus ;  Devonian  ;  showing  spini- 

form cardinal  angles 78 

61.  Illcenus  (Octillcenus)  Hisingeri,  Ordovician,  Bohemia;  a  trilobite; 

showing   spiniform    pleural    extremities    of    first   thoracic 
segment 79 

62.  Cheirurus     insignis,    Silurian,    Bohemia ;     pygidium     and     six 

thoracic  segments 79 

63.  Deiphon  Forbesi,  Silurian,  Bohemia ;    entire  specimen;  showing 

spiniform  pleura  of  segments  corresponding  in  direction  to 
those  of  the  pygidium 79 

64.  Lichas  scabra,  Silurian,  Bohemia ;  pygidium,  with  three  thoracic 

segments  ;  showing  spiniform  ends  of  pleura 79 


Xxii  ILL  USTRA  TIONS 

FICMJRK  PAGE 

65.  Paradoxides  spinosus.  Cambrian,  Bohemia  ;    pygidium  and  six 

free  segments 79 

66.  Female  of  Lernceascus  nematoxys,  a  parasitic  copepod ;  showing 

suppression  of  limbs 85 

67.  Horse-shoe  Crab  (Limulus  polyphemus) ;    showing   telson  spine 

and  abbreviated  abdomen 85 

68.  A  Devonian  phyllocarid  (Echinocaris  socialis) ;  showing  spini- 

form  telson  and  cercopods 85 

69.  Wing  of  Apteryx  australis 85 

70.  Skeleton  of  right  fore  limb  of  the  Jurassic  Dinosaur  Iguanodon 

bernissartensis ;  showing  suppressed  first  digit 85 

71.  Leaf  of  Ratan  (Dcemonorops  hygrophilus) 89 

72.  Leaf  of  Ratan  (Desmoncus  polycanihus) 89 

73.  Bramble  (Rubus  squarrosus^) 89 

74.  Diagram  and  table ;  showing  correlation  of  stages  and  conditions 

of  development  in  the  spinose  individual,  in  its  ancestry, 

and  in  time 100 

75.  Table  of  geological  distribution  of  Trilobita 133 

76.  Agnostus  nudus  Beyrich 177 

77.  Agnostus  rex  Barrande 177 

78.  Trinucleus  ornatus  Sternberg 177 

79.  Hydrocephalus  saturnoides  Barrande 177 

80.  Hydrocephalus  carens  Barrande 177 

81-83.    Olenellus  (Mesonacis)  asaphoides  Emmons 177 

84.    Geological  range  and  distribution  of  Arthropoda 184 

85-94.    Cistella  neapolitana  Scacchi 248-250 

95-98.    SpirorUs  borealis  Daudin 253 

99,  100.    Cistella  neapolitana  Scacchi 254 

101-107.    Thecidium  (Lacazella)  mediterraneum  Risso 258 

108-113.    Cistella  neapolitana  Scacchi       ....          261 

114.  Delthyrium  of  young  Rhynchonella,  without  deltidial  plates  .     .     262 

115.  The  same  at  a  later  stage,  with  two  triangular  deltidial  plates    .     262 

116.  The  same  after  completed  growth  ;  showing  joining  of  deltidial 

plates,  and  limitation  of  pedicle-opening  to  ventral  beak       .     262 

117.  Dorsal   view   of    Magellania    flavescens ;    showing    completed 

deltidial  plates  (del) 262 

118.  The  same;  profile 262 

119.  Dorsal  view  of  umbonal  portion  of  adult  Terebratulina  septen- 

trionalis,  with  shell  removed  by  acid  ;  showing  slight  sec- 
ondary extension  of  ventral  mantle  around  pedicle  .  .  .  262 

120.  Dorsal  view  of  umbonal  portion  of  Magellania  flavescens,  with 

the  shell  removed  by  acid ;  showing  the  complete  envelop- 
ment of  base  of  pedicle  by  secondary  expansions  from  ven- 
tral mantle,  and  consequent  production  of  deltidial  plates 
filling  delthyrium  except  at  pedicle-opening 262 


ILL  US  TRA  TIONS  xxiii 

FIGURE  PAGE 

121.  Stages  of  growth  of  the  lophophore  in   Thecidea,  Cistella,  and 

Megathyris 279 

122.  Stages  of  growth  of  the  lophophore  in  the  Terebratellidse  and 

Terebratulidae 280 

123.  Metamorphoses  of  the  brachidium  in  Dielasma  turgidum  .     .     .  281 

124.  Early  stages  of  lophophore  of   Glottidia  and  adult  brachia  in 

Lingula  and  Hemithyris 282 

125.  Metamorphoses  of  brachidium  of  Zygospira  and  adult  brachid- 

ium of  Rhynchospira 284 

126-128.    Development  of  internal  apparatus  in  Homceospira  evax      .  365 

129.  Deltidial  development  in  Spirifer 384 

130.  Deltidial    development   in    1,    2,    Spiriferina  pinguis   Deslong- 

champs ;  3,  4,  Spiriferina  Walcotti  Sowerby;  5,  Spiriferina 

rostrata  Schlotheim 393 

131.  BiloUtes  varicus  Conrad ;  ventral  area 402 

132.  Genesis  of  Bilobites  ...  404 


I 

GENERAL  EVOLUTION 

1.    THE  ORIGIN  AND  SIGNIFICANCE  OF  SPINES 

i 


STUDIES  IN  EVOLUTION 


GENERAL  EVOLUTION  V  A  L  i FO 

1.    THE   ORIGIN  AND  SIGNIFICANCE   OF  SPINES 

A  STUDY  IN  EVOLUTION* 

(PLATE  I) 

INTRODUCTION 

THE  presence  of  spines  in  various  plants  and  animals  is, 
at  times,  most  obvious  to  all  mankind,  and  not  unnaturally 
they  have  come  to  be  regarded  almost  wholly  in  the  light 
of  defensive  and  offensive  weapons.  Their  origin,  too,  is 
commonly  explained  as  due  to  the  influence  of  natural  selec- 
tion, resulting  in  the  greater  protection  enjoyed  by  spiniferous 
organisms.  But  when,  upon  critical  examination,  it  is  seen 
that  some  animals  are  provided  with  spines  which  apparently 
interfere  with  the  preservation  of  the  individual,  that  other 
animals  develop  spines  which  cannot  serve  any  purpose  for 
protection  or  otherwise,  and  that  spines  themselves  are  often 
degenerate  or  suppressed  organs,  then  it  becomes  evident  that 
the  spinose  condition  may  have  other  interpretations  than  the 
single  one  of  protection. 

The  object  of  this  article  is  to  make  a  few  observations 
on  spinosity,  especially  among  invertebrate  animals,  and  to 
endeavor  to  arrive  at  some  general  conclusions  relating  to  the 
origin  and  significance  of  this  condition.  It  is  believed  that 
the  results  have  a  broader  application  than  is  at  first  apparent, 
and  underlie  important  laws  and  principles  of  organic  evolu- 
tion. In  closely  related  species,  the  presence  or  absence  of 

*  Amer.  Jour.  Sci.  (4),  VI,  1-20,  125-136,  249-268,  329-359,  pi.  i,  1898. 


STUDIES  IN  EVOLUTION 

spines  seems  in  itself  a  trivial  character,  indicating  at  best 
only  specific  differences,  yet  it  will  be  shown  that  the  spines 
are  often  the  expression  of  important  vital  adjustments  and 
conditions,  and  are  not  merely  external  features  of  the  same 
value  a&  color  and  many  other  skin  or  superficial  characters. 
As  will  be  indicated  later  on,  spines  may  also  arise  through 
'the  operations  of  a  number  of  forces  and  conditions,  and  it 
may  well  be  asked,  therefore,  Do  spines  have  any  profound 
significance?  It  must  be  granted  at  the  outset  that  apart 
from  other  characteristics,  or  when  regarded  as  simple  spini- 
form  extensions  of  certain  tissues  or  organs,  they  have  no 
such  value  or  meaning.  How,  then,  should  they  be  con- 
sidered ?  The  reply  is  evident :  Their  importance  lies  not  in 
what  they  are,  but  in  what  they  represent.  They  are  simply 
prickles,  thorns,  spines,  or  horns ;  they  represent,  as  will  be 
shown,  a  stage  of  evolution,  a  degree  of  differentiation  in  the 
organism,  a  ratio  of  its  adaptability  to  the  environment,  a 
result  of  selective  forces,  and  a  measure  of  vital  power. 

After  studying  numerous  organisms,  the  writer  is  led  to 
believe  that  in  every  case  no  single  reason  is  sufficient  to 
account  for  this  spinose  condition.  The  original  cause  may 
not  be  operative  through  the  entire  subsequent  phylogeny, 
so  that  spines  arising  from  external  stimuli  and  then  serving 
important  defensive  purposes  may  at  a  later  period  practi- 
cally lose  this  function;  or  spines  may  become  more  and 
more  developed  simply  by  increasing  diversity  of  growth 
forces,  or  through  the  multiplicity  of  effects.  In  this  way 
causes  may  follow,  overlap,  or  even  coincide  with  each  other ; 
but  in  interpreting  special  cases  the  problems  involved  may 
be  quite  complicated  and  often  obscure. 

In  reviewing  the  development  of  animal  life  from  the 
earliest  Cambrian  to  the  present,  one  cannot  avoid  being 
impressed  by  the  groups  of  spinose  forms  which  appear  here 
and  there  throughout  geologic  time,  and  give  a  special  phase 
to  contemporary  faunas.  Tracing  these  one  by  one  through 
their  geological  development,  it  is  noticed  that  each  group 
began  its  history  in  small,  smooth,  or  unornamented  species. 


ORIGIN  AND  SIGNIFICANCE   OF   SPINES  5 

As  these  developed,  the  spinose  forms  became  more  abundant 
until  after  the  culmination  of  the  group  is  reached,  when  this 
type  either  became  extinct  or  was  continued  in  smaller  and 
less  specialized  forms.  In  applying  this  principle  to  any 
order  of  plants  or  animals,  several  precautions  are  necessary. 
The  estimate  must  be  based  approximately  upon  the  general 
average  of  the  totality  of  specific  characters,  whether  a  genus, 
family,  order,  or  even  a  class  is  being  considered.  A  short- 
lived family  or  genus,  or  the  terminal  members  of  specialized 
groups,  therefore,  cannot  be  taken  as  representing  the  develop- 
mental status  of  the  larger  divisions,  because  they  culminated 
and  disappeared  independently  of  the  culmination  of  the  class 
to  which  they  belong.  On  a  small  scale,  however,  each 
epitomizes  the  rise  and  decline  of  the  larger  group,  and  the 
principles  of  correlation  commonly  applied  in  ontogeny  and 
phylogeny  can  likewise  be  used  in  the  study  of  spines  and 
spiniferous  species,  with  equally  exact  results,  whenever  the 
principal  factors  are  understood. 

Law  of  Variation. 

Before  undertaking  any  general  or  special  examination  of 
the  life  histories  and  interpretation  of  spinose  organisms,  it 
is  desirable  to  consider  briefly  some  of  the  biogenetic  prin- 
ciples which  are  considered  to  bear  directly  on  the  problems 
here  under  discussion. 

First  among  these  is  the  law  of  variation  or  change,  which 
is  so  generally  recognized  as  to  require  but  the  briefest 
restatement. 

The  organic  as  well  as  the  inorganic  world  is  subject  to 
all  the  forces  of  nature,  internal  and  external,  molecular  and 
molar,  and  even  a  partial  stability  is  gained  only  through  a 
regulated  adjustment.  In  organisms  this  change  is  momen- 
tary and  persistent,  while  in  most  inorganic  substances  it  is 
slow  and  intermittent.  The  results  of  this  continual  read- 
justment constitute  modification,  which  may  be  progressive 
or  regressive,  continuous  or  discontinuous  (in  the  sense  of 
accelerated,  uniform,  or  retarded).  They  are  everywhere 


6  STUDIES  IN  EVOLUTION 

present  and  the  causes  always  operative.  Throughout  life 
the  individual  changes,  and  in  addition  varies  from  all  other 
individuals.  The  family  also  changes  with  time,  and  like- 
wise differs  from  other  families.  Variation  is  everywhere 
present.  Moreover,  it  is  generally  accepted,  and  is  so  taken 
here,  that  in  its  results  this  variation  is  not  haphazard, 
but  is  normally  in  accordance  with  certain  demands  or  in 
harmony  with  certain  surroundings.  Whether  an  organism 
itself  tends  to  vary  in  all  directions,  or  is  chiefly  subject  to 
modifications  from  external  forces,  does  not  alter  the  preced- 
ing statement. 

Cope  u  has  considered  variation  as  either  physico-chemical 
(molecular)  or  mechanical  (molar).  The  influence  of  the 
first  is  known  as  physiogenesis  and  of  the  second  as  kineto- 
genesis.  In  the  animal  kingdom  the  potency  of  kinetogenesis 
is  greater  as  an  efficient  cause  of  evolution;  while  in  the 
vegetable  kingdom  physiogenesis  is  apparently  of  more 
importance. 

The  tendency  of  variation  is  always  in  the  direction  of  the 
establishment  of  an  equilibrium  between  the  organism  and  its 
environment.  However,  the  laws  of  the  development  of  the 
earth  preclude  the  possibility  of  a  constant  environment,  and 
therefore  a  perfect,  permanent,  and  uniform  equilibrium 
between  life  and  surroundings  is  unattainable. 

The  manner  of  variation  is  clearly  defined  as  progressive 
and  regressive.  Progressive  variation  is  one  of  the  essential 
factors  of  evolution,  while  regressive  variation  is  towards 
dissolution.  Since  the  main  history  of  life  is  told  through 
processes  of  the  former,  progressive  variation  is  far  greater 
in  importance ;  while,  in  general,  regressive  variation  can  be 
applied  only  to  late  periods  in  the  history  of  groups  or  forms 
now  in  their  decadence,  or  to  others  which  in  past  times  have 
suffered  decline  and  extinction. 

The  summary  of  the  operation  of  the  law  of  multiplication 
of  effects,  as  given  by  Herbert  Spencer,66  may  well  be  stated 
here,  as  it  emphasizes  one  of  the  principles  through  which 
spines  have  originated. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  7 

"It  manifestly  follows  that  a  uniform  force,  falling  on  a 
uniform  aggregate,  must  undergo  dispersion ;  that  falling  on 
an  aggregate  made  up  of  unlike  parts,  it  must  undergo  dis- 
persion from  each  part,  as  well  as  qualitative  differentiations ; 
that  in  proportion  as  the  parts  are  unlike,  these  qualitative 
differentiations  must  be  marked;  that  in  proportion  to  the 
number  of  the  parts,  they  must  be  numerous;  that  the 
secondary  forces  so  produced  must  undergo  further  trans- 
formations while  working  equivalent  transformations  in  the 
parts  that  change  them ;  and  similarly  with  the  forces  they 
generate.  Thus  the  conclusions  that  a  part-cause  of  evolu- 
tion is  the  multiplication  of  effects,  and  that  this  increases  in 
geometrical  progression  as  the  heterogeneity  becomes  greater, 
are  not  only  to  be  established  inductively,  but  are  deducible 
from  the  deepest  of  all  truths." 

Modification,  therefore,  may  properly  include  the  results  of 
the  multiplication  of  effects.  Furthermore,  from  a  knowl- 
edge of  the  life  history  of  the  organic  world,  it  is  known  that 
this  change  has  been  progressive,  resulting  in  the  evolution 
of  the  higher  from  the  lower,  of  the  complex  from  the  simple, 
and  of  the  definite  from  the  indefinite. 

It  must  now  be  asked,  Is  the  amount  of  variation  with- 
out limit  or  is  it  restricted  within  bounds  which  can  be 
determined?  As  far  as  can  be  seen,  the  limitations  of  the 
forms  of  species  of  animals  and  plants  end  only  with  the 
aggregate  number  of  possibilities  within  the  functional  scope 
of  the  organism.  Beyond,  in  either  direction,  is  death,  and 
a  passage  from  the  organic  into  the  inorganic.  The  restric- 
tions of  variation  are  chiefly  those  of  temperature,  pres- 
sure, motion,  light,  space,  time,  and  matter.  Within  certain 
limits,  these  clearly  bound  the  horizon  of  known  possible 
life.  Further,  the  material  constitution  of  the  organic 
world  is  naturally  subject  to  ordinary  mechanical  and  chemi- 
cal laws. 

If,  instead  of  the  preceding,  general,  and  therefore  rather 
abstract  statements  of  the  limits  of  variation,  the  subject 
is  considered  from  the  concrete,  objective  side,  the  limits 


8  STUDIES   IN  EVOLUTION 

between  which  are  found  all  the  variations  actually  presented 
by  any  character  or  set  of  characters,  in  the  animal  or  the 
vegetable  kingdoms,  can  at  once  be  determined.  The  fact 
that  the  organic  world  can  be  divided  into  kingdoms,  sub- 
kingdoms,  classes,  orders,  etc.,  and  definitions  of  the  divi- 
sions given,  in  itself  furnishes  sufficient  evidence  that  these 
have  been  the  limits  of  organic  change,  at  least  under  present 
terrestrial  conditions.  This  does  not  imply  that  the  phylog- 
enies  of  groups  of  animals  and  plants  do  not  converge  and 
coalesce,  and  join  larger  and  larger  phyla  in  past  ages,  so 
that  the  gaps  between  unlike  forms  are  gradually  filled  by 
complete  series.  It  does,  however,  express  the  definite 
heterogeneity  of  the  results  of  development. 

For  the  sake  of  illustrating  an  extreme  range  of  variation, 
it  will  be  granted  that  the  assemblage  of  characters  by  which 
a  mammal  is  now  recognized  precludes  mammalian  variation 
into  a  cold-blooded,  non-vertebrate,  lungless  animal.  Like- 
wise the  mammalian  skeleton  cannot  be  siliceous  or  chitinous. 
Externally  mammals  may  be  smooth,  hairy,  scaly,  or  plated, 
but  not  feathered.  There  may  be  found  numerous  gradations 
from  the  smooth  to  the  plated  state,  and  a  great  range  of 
variation  in  each  type  of  epidermal  structure.  In  vertebrate 
animals  generally,  the  hair  may  vary  in  length,  in  fineness, 
in  color  and  shape ;  it  may  form  bristles,  or  spines,  or  feathers ; 
and,  as  a  skin  character,  it  is  related  to  horn-sheaths,  hoofs, 
nails,  claws,  scales,  and  teeth.  These  constitute  the  limits 
of  modification  in  epidermal  or  exoskeletal  growths.  The 
types  are  few,  but  the  variety  in  each  is  almost  infinite. 
The  variation  may  be  seen  in  individuals,  but  becomes  greater 
in  species,  and  increases  still  more  in  larger  groups.  The 
gradations  are  numerous  between  the  hair  of  a  Beaver  and 
the  spines  of  a  Porcupine;  between  the  horns  of  the  Giraffe, 
Rhinoceros,  and  Antelope ;  between  the  nails  of  Man  and  the 
claws  of  the  Carnivora;  and  between  the  teeth  of  a  Dog-fish 
and  those  of  a  Tiger. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  9 

Definition  of  Terms. 

In  the  beginning  it  is  well  to  understand  the  meaning  and 
extent  of  the  terms  included  under  the  comprehensive  word 
spine.  In  a  general  sense,  spine  is  here  used  to  cover  any 
stiff,  sharp-pointed  process.  A  prickle  is  restricted  in  use 
to  the  small,  sharp-pointed,  conical  projections  which  are 
purely  cuticular;  as  in  the  Rose  and  Blackberry.  A  thorn 
is  a  sharp  process  on  plants,  usually  representing  a  branch 
or  stem.  A  horn  is  an  excrescence  on  the  head  of  cer- 
tain animals,  and  is  properly  hollow.  An  antler  is  a  solid 
bony  process,  usually  deciduous,  and  generally  confined  to 
the  male;  as  in  the  Deer  or  Elk.  A  spur  is  a  term  applied 
to  the  claw-like  process  on  the  legs  and  wings  of  some  birds, 
and  on  the  hind  legs  of  Ornithorhynchus  and  Echidna. 

The  word  spine,  therefore,  is  most  comprehensive,  and  is 
here  intended  to  include  the  modified  hairs  of  the  Echidna 
and  Porcupine ;  the  sharp,  prickly  scales  of  the  Horned  Toad 
(Phrynosoma);  the  pointed  spiniform  projections  on  the  shells 
of  Mollusca;  the  spinous  prominences  on  the  test  of  Crus- 
tacea and  insects ;  the  fin  spines  as  well  as  those  on  the  oper- 
cula  and  scales  of  fishes ;  the  generally  movable  processes  of 
Echinodermata ;  the  projecting  rays  and  processes  of  Radio- 
laria,  etc.,  etc.  The  vertebral  column  and  also  the  processes 
from  the  separate  vertebrae  are  known  as  spines,  but  as  these 
are  distinctly  internal  structures,  they  will  not  be  considered 
in  this  connection. 

In  nearly  all  classes  of  organisms  spines  have  been  devel- 
oped independently,  and  simply  represent  cases  of  parallel 
development  of  similar  structures  or  morphological  equiva- 
lents. They  possess  analogy  of  form  without  necessary 
homology  of  structure,  and  accordingly  have  no  common 
phylogenetic  connection.  Therefore,  if  the  relationships  be- 
tween the  smooth  and  spinose  forms  belonging  to  any  group 
of  animals  or  plants  can  be  traced,  and  the  simplest  and  most 
primitive  condition  in  each  case,  as  well  as  the  highest  stage 
of  progressive  development,  can  be  ascertained,  their  relative 


10  STUDIES  IN  EVOLUTION 

significance  from  an  evolutionary  standpoint  may  be  confi- 
dently determined. 

Growth  of  a  Spine. 

The  growth  of  a  spine  is  either  direct  an .   progressive,  or 
indirect  and  regressive.     It  is  direct  when  it  is  developed  by 

12345 


FIGURES  1-5.  —  Different  stages  in  the  growth  of  a  spine.     1,  plane  surface  ; 
2,  slight  elevation  ;  3,  node  ;  4,  short  spine  ;  5,  completed  simple  spine. 

the  addition  of  new  tissue.  In  this  way  growth  is  attained 
in  the  antlers  of  a  Deer,  the  horns  of  a  Cow,  the  ordinary 
spines  of  Brachiopoda,  Mollusca,  and  Crustacea, 
and  in  other  similar  examples  covering  the  major- 
ity of  cases.  Growth  is  indirect,  however,  when 
the  spine  represents  atrophy  or  suppression  of  an 
organ  through  the  loss  of  its  accessory  parts  ;  as 
in  the  thorns  of  the  Locust  and  the  Barberry, 
the  spiniform  termination  of  the  stems  of  the 
Pear,  or  the  spurs  on  the  Python. 

The   direct  development  of  a  spine   is  essen- 
tially the  same  process  in  all  cases.     At  a  given 
point  on  the  surface  of  an  organism,  there  first 
appears  a  slight  elevation,  which  becomes  higher 
and  higher,  and  is  usually  conical  in  form.     This 
cone  represents  the  simplest  type  of  spine;  and 
FIGURE  6.  among  animals  and  plants  most  spines  conform 
a  single  radi-  to  this  primitive  pattern  (figures  1-5). 

Often  there  are  various  kinds  of  surface  orna- 


w-  ments,  which  by  growth  and  differentiation  de- 
ofg  Ha^ned  velop  into  spines.    By  rhythmic,  alternating  areas 


spines.  Of  accelerated  or  retarded  growth,  the  concentric 

laminae  on  many  molluscs  may  produce  spines,  as  shown  in 
figure  26.      In  the  same  way  the  radiating  ridges  may  be 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


11 


diversified  into  a  row  of  spines,  as  represented  in  figure  6. 
Further,  the  surface  may  be  reticulate,  with  longitudinal 
and  transverse  lines,  and  at  the  points  of  intersection,  nodes 
and  often  spines  are  formed  after  the  manner  shown  in 
figures  7-12.  The  longitudinal  or  vertical  lines  may  become 
obsolete,  leaving  the  spines  to  be  borne  on  the  transverse  or 


10 


11 


I 

, 

t 

j 

{ 

i 

f 

i 

( 

i 

i 

/ 

f\ 

, 

, 

i 

12 

/  y  y  /  y 
/  y  y  y  / 
9  i  t.n 

(  y  y  /  i 
lint 

FIGURES  7-12.  — Diagrams  ;  showing  growth  and  differentiation  of  ornament 
into  spines.  7,  surface  with  parallel  lines  ;  8,  surface  with  regular  reticulate  lines ; 
9,  same,  with  spines  developed  at  the  points  of  intersection ;  10,  same,  with  the 
vertical  lines  obsolete,  but  still  represented  by  the  vertical  rows  of  spines;  11, 
same,  with  the  horizontal  lines  obsolete,  but  still  represented  by  the  horizontal 
arrangement  of  the  spines  ;  12,  same,  with  all  lines  obsolete,  but  both  series 
represented  by  the  vertical  and  horizontal  arrangement  of  the  spines. 

horizontal  lines  (figure  10).  In  other  cases  the  horizontal 
lines  disappear,  leaving  the  spines  on  the  vertical  lines 
(figure  11).  Finally,  both  horizontal  and  vertical  lines  be- 
come obsolete,  and  then  only  the  spines  remain,  as  shown  in 
figure  12. 

The  indirect  production  of  spines  is  not  always  evident,  for 
if  the  ontogeny  or  phylogeny  of  the  individual  is  unknown, 


12 


STUDIES  IN  EVOLUTION 


13 


its  direct  or  indirect  development  cannot  be  determined.  An 
excellent  example  of  indirect,  or  regressive,  growth  of  spines 
is  afforded  in  the  common  Barberry  {Berberis  vulgaris),  on 
the  summer  shoots  of  which  are  shown  most  of  the  gradations 
"between  the  ordinary  leaves,  with  sharp  bristly  teeth,  and 
leaves  which  are  reduced  to  a  branching  spine  or  thorn.  The 
fact  that  the  spines  of  the  Barberry  produce  a  leaf-bud  in 

their  axil   also  proves   them  to 
be  leaves"24  (figure  13). 

It  should  be  noted  that  the 
process  of  spine  development 
illustrated  in  Spondylus  (figure 
M  KY*  14)  is  directly  opposed  to  that 
of  the  Barberry.  In  the  former 
the  initial  growth  is  smooth,  then 
faint,  concentric,  and  radiating 
lines  appear,  which  gradually 
grow  stronger,  developing  more 
or  less  regular  inequalities ;  and 
by  the  excessive  growth  of  these 
variations  spines  are  formed.  In 
FIGURE  is.  —  Summer  shoot  of  the  Barberry  there  are  at  first 

Barberry  ;  showing  the  gradations  normal  leaves,  which  are  followed 
between  leaves  and  spines.  The  ,  ,  ,  ,  , 

arrow  indicates  the  direction  of  bJ  others  more  and  more  toothed 
growth.  (After  Gray.)  and  bristly,  until  the  leaf  is  rep- 


imperialis;  showing  the  series  of  while  finally  Spines  Only  are 
spines.  The  arrow  indicates  the  formed.  The  Spondylus  repre- 
direction  of  growth.  .  .  . 

sents   a  progressive   increase  in 

growth  to  produce  the  spines,  while  the  Barberry  exhibits  a 
progressive  decrease  of  growth,  or  an  "ebbing  vitality,"  as  it 
has  been  termed  by  Geddes.20 

The  spines  are  the  final  results  of  both  the  direct  and 
indirect  modes  of  production;  the  direct,  through  a  process 
of  building  on  new  tissue,  and  the  indirect,  through  a  process 
of  dwindling  away  to  all  but  the  axial  elements.  These 
differences  are  graphically  expressed  in  figures  13  and  14. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


13 


Attention  should  be  called  to  the  four  kinds  of  spine 
production  in  different  organisms.  (1)  In  the  Radiolaria, 
Echinoidea,  the  Giraffe,  Cattle,  and  the  Rhinoceros,  the 
spines  or  horns  are  persistent,  and  grow  by  additions  to  the 
original  structure.  The  new  tissue  may  be  superficial,  sub- 
terficial,  interstitial,  or  formed  by  synchronous  resorption 
and  growth.  (2)  In  the  Crustacea  and  Articulata  generally, 
and  in  the  Deer,  Elk,  etc.,  the  spines  are  moulted,  or  shed, 
periodically.  In  their  various  stages,  these  types  (1  and  2) 


15 


FIGURE  15.  —  Example  of  spine  growth  by  simple  increscence.  Horn  (left) 
and  horn-core  (right)  of  Ox.  (After  Owen.) 

FIGURE  16.  —  Stages  of  spine  growth  by  successive  replacement.  Antler  series 
of  Red  Deer,  at  ages  of  1,  2,  3,  etc.,  years.  (After  Owen.) 

FIGURE  17.  —  Stages  of  spine  growth  by  serial  repetition.  Profile  of  a  series 
of  spines  on  one  of  the  primary  radii  of  Spondylus  imperialis. 

FIGURE  18.  —  Stages  of  spine  growth  by  decrescence.  Transformation  of 
leaves  into  spines  in  Berberis  vulgaris.  (After  Gray.) 

can  be  studied  only  by  means  of  separate  specimens  con- 
secutive in  age,  or  by  observing  the  metamorphoses  in  one 
individual.  (3)  In  the  shells  of  Brachiopoda  and  Mollusca, 
the  stages  of  growth  of  the  individual  are  generally  retained 
throughout  life,  and  the  successive  development  of  spines 
may  be  studied,  therefore,  in  a  single  example.  (4)  Spines 
produced  by  suppression,  as  in  the  Barberry,  express  their 
origin  through  a  series  of  gradations  between  separate  parts ; 
while  in  others  suppression  is  brought  about  by  the  loss  of 
structures. 


14  STUDIES  IN  EVOLUTION 

The  first  type  mentioned  develops  horns  or  spines  by 
simple  increscence  (figure  15);  for  example,  the  Ox:  the 
second,  by  successive  replacement  (figure  16);  as  in  the 
Deer:  the  third,  by  serial  repetition  (figure  17);  for  example, 
Spondylus :  the  fourth,  by  decrescence  (figure  18);  for 
example,  the  Barberry. 

Localized  Stages  of  Growth.  —  By  the  multiplication  of  sur- 
face ornaments  through  the  process  of  interpolation,  many 
Mollusca  present  stages  of  spine  development  in  two  direc- 
tions. (1)  The  normal  series  is  represented  by  the  succession 
of  spines  along  a  single  sector  of  growth.  For  instance,  in 
the  radial  plications  of  a  Spondylus  or  Lima,  the  earliest  and 
primitive  spines  are  found  near  the  beak,  while  those  on  the 
ventral  border  of  an  adult  specimen  are  the  latest  and  most 
highly  developed  (figure  30).  These  successive  stages,  there- 
fore, are  in  the  direction  of  growth,  and  may  be  called  longi- 
tudinal. (2)  By  the  radial  divergence  of  the  ribs  or  plications 
and  the  interpolation  of  additional  ones  at  various  intervals, 
as  many  transverse  compound  series  of  spines  finally  appear 
along  the  periphery  as  there  are  primary  radii.  Hence,  in  a 
given  case,  there  may  be  two  radii  continuing  to  the  beak, 
then  by  interpolation  there  are  successively  5,  11,  23,  etc., 
radii,  the  highest  number  being  found  at  the  periphery 
(figures  19,  20).  Moreover,  by  taking  the  distal  spines  on 
these  23  rows,  there  result  the  same  stages  of  spine  develop- 
ment as  shown  in  the  longitudinal  series  along  any  primitive 
plication  (figure  20).  A  pelecypod  shell  like  Spondylus  is 
here  used  to  illustrate  this  process,  but  the  application  may 
also  be  made  to  the  Brachiopoda  as  well  as  to  the  conical 
non-coiled  Gastropoda.  In  a  coiled  form  like  a  cephalopod 
or  an  ordinary  gastropod,  the  longitudinal  lines  would  follow 
the  whorls  spirally,  and  the  transverse  lines  would  corre- 
spond to  the  lines  or  increments  of  growth  of  the  shell. 
Species  in  which  the  radii  are  all  introduced  at  an  early 
stage  of  growth  (many  species  of  Cardium,  Pecten,  Lima) 
or  in  which  the  radii  multiply  by  regular  dichotomy  would 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


15 


show,  of  course,  only  the  longitudinal  series,  for  at  the 
margin  of  the  shell  the  radii  would  be  of  the  same  size  and 
age,  and  the  spines  uniform. 

The  foregoing  example  illustrates  an  important  principle 


19 


20 


44 

40*  • 

ililiiifi i 


FIGURE  19.  —  Sector;  showing  in  diagram  the  multiplication  of  radiating 
lines  by  interpolation.  The  two  primary  radii  (1,1)  are  the  only  ones  continuing 
through  the  whole  four  zones.  The  first  zone  lias  2  radii ;  the  second,  5  ;  the  third, 
1 1 ;  and  the  fourth,  23. 

FIGURE  20.  —  Profiles  of  the  spines  produced  on  the  various  radii  at  the  four 
zones  ;  as  indicated  in  the  preceding  figure.  A,  the  spines  on  the  two  primary 
radii  of  the  first  zone ;  B,  the  spines  on  the  second  zone,  showing  the  growth  of 
those  on  the  two  primary  radii  (1,  1),  and  the  small  spines  on  the  newly  interpo- 
lated radii  (2,  2,  etc.)  ;  C,  the  spines  on  the  radii  in  the  third  zone ;  D,  the  spines 
at  the  bottom  of  the  fourth  zone.  The  two  large  compound  spines  are  on  the  two 
primary  radii.  Their  development  may  be  traced  by  following  them  through  A, 
B,  C,  to  D.  The  next  three  longest  spines  (2,  2,  2)  are  tricuspid,  and  represent 
the  stage  of  spine  development  attained  by  the  spines  on  the  radii  which  were 
interpolated  on  the  second  zone.  The  next  six  smaller  spines  (3,  3,  3,  etc.)  are  on 
radii  which  were  introduced  on  the  third  zone.  The  twelve  small  spines  (4,  4,  4, 
etc.)  are  on  the  radii  introduced  on  the  fourth  zone.  Thus  there  are  four  stages 
of  spine  growth  shown  on  the  lower  margin  of  the  fourth  zone,  and  these  corre- 
spond to  the  four  stages  exhibited  by  the  series  of  spines  on  one  of  the  primary 
radii  running  through  the  four  zones. 

of  ontogeny ;  namely,  that  in  organisms  which  repeat  various 
parts  during  their  growth,  these  parts  will  develop  or  pass 
through  a  series  of  stages  corresponding  to  the  initial  and 
subsequent  stages  of  the  parts  repeated.  In  this  way  struc- 


16  STUDIES  IN  EVOLUTION 

tures  appearing  late  in  the  ontogeny  of  the  individual 
will  present  primitive  infantile  and  adolescent  characters. 
Further  development,  if  such  takes  place,  will  pass  through 
a  progressive  series  of  ontogenetic  changes,  and  if  the  stages 
of  growth  are  by  serial  repetition  and  thus  are  retained  in 
the  part,  it  will  be  found  that  such  stages  can  be  correlated 
with  those  appearing  early  in  the  life  or  history  of  the  indi- 
vidual. Therefore,  in  studies  of  this  kind,  it  is  possible  to 
take  a  structure  appearing  at  maturity,  and  from  it  deduce  or 
predicate  as  to  what  were  some  of  the  early  characteristics 
of  the  whole  individual.  This  principle  is  termed  localized 
stages  of  growth  by  Jackson,37  and  was  first  noticed  by  him 
in  some  investigations  on  Echinodermata. 

Compound  Spines. 

A  simple,  sharp,  conical  process  expresses  only  the  primi- 
tive type  of  spine.     In   plants  and  animals  it  is  the  most 


FIGURE  21.  —  Simple  spine. 

FIGURE  22.  —  Spine,  with  lateral  spinules. 

FIGURE  23.  —  Spine,  with  forked  apex  and  lateral  spinulose  spinules. 

common  form  found,  and  is  the  first  stage  of  spine  differen- 
tiation. From  this  type  the  myriad  forms  of  spines  known 
in  the  organic  world  are  produced  by  almost  insensible 
gradations.  It  is  needless  to  attempt  a  detailed  description 
of  this  infinite  variety;  but,  as  a  single  illustration,  some  of 
the  leading  forms  of  spine  differentiation  among  the  Radio- 
laria  are  here  shown  (Plate  I).  These  figures  are  taken 
from  Haeckel's  "Report  on  the  Radiolaria,"26  and  generally 
represent  enlargements  of  from  100  to  400  diameters.  Prob- 
ably no  other  class  of  organisms  presents  greater  variety,  and 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  17 

many  of  the  forms  are  repeated  again  and  again,  not  only 
in  various  species  of  this  group,  but  elsewhere  both  in  the 
animal  and  vegetable  kingdoms. 

Whenever  the  development  of  a  compound  spine  can  be 
studied,  it  shows  a  gradual  progress  from  the  simple  to  the 
complex  (figures  21-23).  The  antlers  of  the  Red  Deer 
(Cervus  elaphus)  furnish  a  familiar  example.  Fawns  of  the 
first  year  have  antlers  with  only  a  single  prong,  a  short  front 
tine  being  added  the  second  year;  then  "year  by  year  as 
they  are  renewed  they  acquire  a  greater  and  still  greater 
number  of  tines  and  branches,  till  they  finally  attain  the 
complete  stage,  when  their  owner  is  termed  a  '  royal  hart'  "44 
(figure  16).  Although  somewhat  conventionalized,  the  pri- 
mary series  of  spines  on  the  Spondylus  shown  in  figure  20 
exhibits  the  passage  from  simple  to  compound  forms.  An 
inspection  of  many  species  of  Murex  will  show  the  stages  in 
series  presenting  a  greater  complexity. 

After  spine  development  has  reached  its  maximum  growth 
and  differentiation,  evidence  of  old  age  may  be  exhibited  in 
two  ways:  (a)  The  spines  may  be  reduced  by  resorption, 
decay,  or  abrasion,  and  finally  become  obsolescent;  or  what 
is  of  greater  import  (5),  they  may  gradually  cease  to  be 
developed,  as  is  especially  shown  in  organisms  in  which  spine 
growth  is  by  serial  repetition.  Thus,  in  Spondylus  calcifer, 
a  young  individual  measuring  about  two  inches  across  has 
marginal  spines  fully  an  inch  in  length.  Even  longer  spines 
are  found  when  the  shell  reaches  a  width  of  four  inches.  On 
attaining  a  maximum  diameter  of  about  six  inches,  spine 
growth  gradually  ceases,  and  the  margin  of  the  valves  is 
entire  and  nearly  smooth.  At  this  stage  shell  secretion  is 
confined  to  excessive  thickening  of  the  valves.  These  senile 
stages  of  spine  growth  will  receive  further  consideration 
under  the  discussion  of  ontogeny  and  phylogeny  of  spinous 
species. 

Application  of  Law  of  Morphogenesis.  —  The  manner  in 
which  spines  arise  from  plane  surfaces,  or  from  the  growth 

2 


18  STUDIES  IN  EVOLUTION 

or  modification  of  superficial  structures,  and  also  through  the 
decadence  of  organs,  has  now  been  noticed.  The  spine  may 
thus  be  taken  as  a  unit  for  comparison,  and  its  various  stages 
of  growth,  which  were  shown  to  have  a  definite  sequence, 
may  be  used  in  correlation  to  determine  relatively  the  degree 
of  spine  specialization  attained  by  any  organism.  Further- 
more, enough  data  have  been  already  given  to  lead  to  the 
suspicion  that  spines  may  represent  the  limits  of  ornamental 
or  superficial  differentiation  or  variation.  At  this  point  in 
the  discussion  this  statement  must  be  considered  as  more 
suggestive  than  conclusive.  The  proof  of  its  reality  will  be 
more  clearly  shown  later  on. 

Ontogeny  of  a  Spinose  Individual. 

With  few  exceptions  the  embryonic  and  larval  stages  of 
all  organisms  are  devoid  of  specialized  surface  features.  In 
other  words  they  are  without  ornament  and  without  weapons. 
The  exceptions  to  this  rule  seem  to  be  readily  explained 
under  the  principles  of  larval  adaptations  and  accelerated 
development.  Cases  of  the  latter  kind,  therefore,  can  hardly 
be  considered  as  exceptions,  since  they  represent,  not  real 
larval  features,  but  former  adult  characters  which  have  been 
pushed  back  or  which  develop  earlier  so  as  to  appear  even- 
tually in  the  larval  or  later  embryonic  stages.  In  the  very 
earliest  stages  of  embryonic  development,  the  truth  of  the 
first  statement  becomes  obvious,  and  accordingly  the  pro- 
tembryonic,  mesembryonic,  metembryonic,  neoembryonic,  and 
typembryonic  stages  are  without  surface  ornaments  or  spines. 

Among  Mollusca,  the  protoconch,  periconch,  and  prodis- 
soconch,  or  the  early  larval  shells,  are  smooth  and  without 
ornament.  Even  the  prodissoconch  of  very  highly  spinose 
species,  as  in  Spondylus,  is  as  smooth  as  that  of  the  plainest 
species  of  Ostrea,  Anomia,  Avicula^  etc.  Likewise,  the  proto- 
conch of  the  most  specialized  or  most  retrograde  cephalopod 
is  perfectly  plain.  In  the  nepionic  stages  the  spiny  Murex 
is  without  spines.  In  the  Brachiopoda  the  protegulum,  or 
early  larval  shell,  is  always  without  sculpture;  while  the 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  19 

nauplius  of  Crustacea  and  the  protaspis  of  Trilobita  are 
generally  spineless.  The  young  of  horned  vertebrates  are 
almost  universally  hornless,  the  Giraffe  being  the  only  mam- 
mal born  with  horns.  The  very  young  seedlings  of  plants 
are  likewise  spineless.  In  insects  the  embryonic  stages 
generally  have  simple  cuticles,  but  in  the  larval  stages  of 
this  class  and  the  Crustacea,  a  great  variety  of  spines  and 
ornamental  characters  is  developed.  Altogether,  it  may  be 
asserted  that  spines  do  not  appear  during  the  embryonic 
stages  of  animals  and  plants,  and  that  their  initial  develop- 
ment is  commonly  post-larval. 

Examples  illustrating  the  ontogeny  of  a  spinose  form  could 
be  multiplied  indefinitely,  and  taken  from  nearly  every  class 
of  organisms.  In  all  cases  practically  the  same  sequence  of 
events  relating  to  the  development  of  spines  would  be  found. 
The  organism  would  first  be  smooth,  without  sculpture  or 
ornament,  like  the  young  of  other  organisms.  At  some  stage 
of  the  ontogeny  the  beginnings  of  spines  would  appear,  and 
develop  first  into  simple,  and  later,  according  to  the  stage  of 
differentiation  attained,  into  compound  spines.  This  pro- 
gression would  finally  reach  the  maximum,  spine  growth 
would  cease,  and  the  surface  of  the  organism  would  inversely 
revert  to  an  early  and  more  primitive  type  without  spines. 
Normally  these  changes  would  represent  the  infantile,  ado- 
lescent, mature,  and  early  and  late  senile  periods  of  the  life 
of  the  organism.  In  some  cases,  however,  the  stages  of  spine 
growth,  or  acanthogeny,  do  not  agree  with  the  ontogeny  of 
the  entire  individual  in  respect  to  time,  and  here  acceleration 
and  the  phylogeny  of  the  species  will  be  found  to  offer  the 
proper  explanation  of  the  divergence. 

As  simple  examples  of  the  ontogeny  of  spiniferous  species, 
the  Mollusca  afford  especial  advantages,  owing  to  the  fact 
already  noticed,  that  the  stages  of  development  are  commonly 
preserved  in  a  single  individual.  In  figure  24  the  larval 
shell,  or  prodissoconch,  of  Pelecypoda,  or  bivalve  shells,  is 
represented,  and  shows  the  usual  type  throughout  a  large 
portion  of  the  class.  The  succeeding  shell  growth  of  the 


20 


STUDIES  IN  EVOLUTION 


dissoconch  is  at  first  generally  smooth,  save  for  the  fine  con- 
centric lines  of  growth  (figure  25).  In  ornamented  or  spinose 
species,  however,  irregularities  in  the  growth  lines  soon  appear 
(figures  26,  27),  and  these  shortly  assume  the  characteristic 
surface  sculpture  of  the  normal  adult.  Thus  the  prodisso- 
conch  of  Avicula  sterna  is  represented  at  £>,  figure  25,  and 
is  followed  by  regular  concentric  growth  during  the  nepionic 


24 


25 


26 


FIGURE  24.  —  Prodissoconch  of  Ostrea  virginiana.     X  43. 

FIGURE  25.  —  Each  stage  of  Avicula  sterna;  p,  prodissoconch.     X  19. 

FIGURE  26.  —  Young  Avicula  sterna ;  showing  the  beginning  of  spine  growth. 
X  3. 

FIGURE  27.  —  Young  Saxicava  arctica.     X  19. 

FIGURE  28.  —  Young  Anomia  aculeata;  prodissoconch  succeeded  by  early 
smooth  and  later  spinous  dissoconch  growth.  X  30.  (Figures  24-28  after 
Jackson.) 

stages.  In  figure  26  the  spiny  characters  of  early  adoles- 
cence are  added  to  the  previous  stages,  and  in  later  stages 
the  spines  become  more  and  more  emphatic. 

In  Spondylus  the  prodissoconch  is  the  same  simple  form, 
and  is  succeeded  by  a  nearly  smooth  Pecten-like  stage,  dur- 
ing which  the  animal  was  free  (figure  29).  After  fixation 
the  growth  is  very  irregular  and  ostrseiform  for  a  time,  until 
the  shell  rises  above  the  object  of  support,  when  all  the  most 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


21 


characteristic  features  of  surface  ornamentation  become  fully 
developed  (figure  30).  As  the  shells  approach  maximum 
growth,  the  spines  gradually  become  shorter,  and  in  old  age 
none  are  developed,  even  those  of  early  growth  being  removed 
by  the  action  of  boring  animals  and  by  solution  (figure  31). 

It  seems  unnecessary  to  increase  the  number  of  examples 
showing  the  ontogeny  of  spinose  individuals.     The  Deer  and 


29 


31 


FIGURE  29.  —  Young  Spondylus  princeps.  Eight  valve  ;  showing  pecteniform 
stage  succeeded  by  ostraeiform  growth.  Taken  from  apex  of  adult  specimen ; 
presented  by  R.  T.  Jackson.  X  3. 

FIGURE  30.  —  Side  view  of  Spondylus  calcifer,  about  one-third  grown;  show- 
ing the  characteristic  spinous  growth.  |. 

FIGURE  31.  —  Side  view  of  Spondylus  calcifer ;  showing  the  greatly  thickened 
right  valve  and  the  entire  absence  of  spines  over  the  whole  shell.  \. 

the  Ox  may  be  again  cited  in  this  connection.  Both  are 
born  without  horns,  but  during  adolescence  the  antlers  of 
the  Deer  become  longer  and  more  complicated  with  each 
renewal,  while  the  horns  of  the  Ox  are  longer  and  more 
twisted.  In  old  age,  when  the  Deer  has  passed  his  prime, 
the  antlers  are  more  obtuse,  and  exhibit  a  tendency  toward 
decline  and  obliteration.  Suppression  of  the  antlers  is  accom- 
plished by  the  removal  of  the  cause  of  antler  growth  and 
specialization,  so  that  the  unsexing  of  the  male  results  in 


22  STUDIES  IN  EVOLUTION 

small  antlers,  which  are  seldom  branched,  and  become  thick- 
ened by  irregular  deposits  of  bone  (Owen 53).  Spines  grow 
during  the  adolescence  of  the  Horseshoe  Crab,  Limulus 
polyphemus,  yet  in  old  age  they  are  obsolescent,  being  repre- 
sented by  rounded  nodes. 

As  examples  illustrating  the  accelerated  development  of 
spines  in  widely  separated  classes,  the  Giraffe  among  mam- 
mals and  Acidaspis  among  Arthropoda  may  be  selected.  The 
Giraffe  represents  the  continuance  of  a  very  primitive  type  of 
horn ;  namely,  one  covered  with  a  hairy  skin.  They  are  never 
shed,  and  are  common  to  both  sexes.  Out  of  this  type  all 
others  found  among  the  Mammalia  have  probably  been  devel- 
oped. The  point  of  interest  here  is  that  the  young  Giraffe  is 
born  with  horns,  and  as  these  could  serve  no  prenatal  purpose, 
it  must  be  concluded  that  the  action  of  accelerated  heredity 
has  pushed  the  development  of  these  organs  so  far  forward 
as  to  cause  them  to  appear  during  foetal  growth. 

The  next  illustration  of  acceleration  is  taken  from  the  Trilo- 
bita.  Acidaspis  is  one  of  the  most  highly  specialized  and 
ornate  genera.  Although  the  larval  forms  of  other  genera  are 
commonly  without  ornament,  yet  in  the  present  genus  the 
protaspis,  or  phylembryonic,  stage  partakes  of  this  specializa- 
tion in  so  far  as  to  develop  minute  spines,  which  later  become 
larger,  more  differentiated,  and  form  a  conspicuous  feature  of 
the  adult.  Other  characters  have  been  likewise  shown  to 
appear  at  an  earlier  period  than  in  other  genera,  and  the  earlier 
inheritance  of  spines  must  be  explained  in  the  same  manner.8 

The  facts,  as  stated,  seem  to  warrant  the  conclusion,  that  in 
spinose  organisms  the  very  young  are  almost  universally  with- 
out spines.  Acceleration  may  occasionally  push  their  devel- 
opment into  the  embryonic  and  larval  stages,  but  ordinarily 
they  are  not  so  subject  to  the  action  of  this  law  as  are  some  of 
the  physiological  and  other  structural  characters.  This  will  be 
explained  as  in  part  due  to  the  lack  of  general  plasticity,  and 
because  differentiated  spine  growth  is  the  progressive  limit  of 
variation.  Therefore  there  are  no  subsequent  characters  to 
displace  them  and  crowd  them  forward  in  the  ontogeny. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  23 

Phylogeny  of  Spinous  Forms. 

To  interpret  phylogeny  in  terms  of  ontogeny,  according  to 
the  law  of  morphogenesis,  or  recapitulation,  is  perhaps  easier 
than  to  trace  a  genetic  sequence  through  a  series  of  forms 
having  a  considerable  geologic  range.  Taking  the  ontogenies 
of  the  animals  already  noticed,  there  is  for  the  Pelecypoda  the 
prodissoconch,  which  is  correlated  by  Jackson  36  with  Nucula, 
and  a  Lower  Silurian  nuculoid  radicle  is  assumed  for  the 
Aviculidse  and  allied  forms.  The  first  dissoconch  growth  pro- 
duces a  shell  resembling  Rhombopteria,  a  Lower  and  Upper 
Silurian  type,  and  this  is  taken  to  represent  the  second  stage 
in  the  phylogeny  of  Avicula  (figure  25),  Anomia,  Spondylus , 
etc.  Continuing  the  development  of  Spondylus,  it  is  found 
by  Jackson  that  it  passes  successively  through  stages  which 
may  be  correlated  with  Pterinopecten  (Devonian),  Avion- 
lopecten  (Devonian),  Pecten  (Carboniferous  ?),  and  Hinnites 
(Trias),  while  finally  it  assumes  true  spondyliform  characters. 
These  correlations  agree  with  the  geologic  sequence  of  the 
genera,  and  are  believed  to  indicate  phylogenetic  relationships. 
It  may  be  further  remarked  that  the  early  species  of  Spondyli 
are  more  truly  pecteniform  and  hinnitiform  than  the  later 
ones.  The  genus  ranges  from  the  Trias  to  the  present. 
Zittel 73  remarks  that  "  the  oldest  species  are  small,  thin-shelled, 
and  seldom  much  ornamented."  Even  in  the  Cretaceous,  the 
majority  of  species  are  not  far  removed  from  Pecten  and 
Hinnites.  During  the  Tertiary  the  irregular,  ostrseiform, 
squamous,  concentric,  and  spinous  growth  becomes  more 
manifest,  and  at  present  most  of  the  species  show  a  great 
development  and  differentiation  of  the  spines. 

Thus,  while  Spondylus  is  normally  considered  as  a  spinose 
genus  and  the  species  are  familiarly  known  as  Spiny  Oysters, 
yet,  as  it  is  traced  back  in  geological  history,  the  forms  become 
less  and  less  spinose,  and  their  affinities  and  appearances  are 
more  and  more  in  accord  with  non-spinose  genera,  until  finally 
the  prototype  is  a  smooth,  simple,  delicate,  unornamented 
shell. 


24  STUDIES  IN  EVOLUTION 

The  simple  antlers  of  the  young  Deer  and  Elk  correspond 
in  type  with  those  of  the  adults  of  the  Middle  Tertiary  Deer 
(Lydekker  44),  and  it  may  be  therefore  assumed  that  the  great 
number  of  branches  and  tines  is  a  modern  development. 
Further  back  in  the  Tertiary  the  ancestors  of  the  Deer  were 
without  antlers,  thus  representing  in  phylogeny  the  new-born 
Deer  of  the  living  type.  These  correlations  are  made  from 
comparisons  of  chronogenesis,  or  development  in  time,  and 
ontogenesis,  or  development  in  the  individual. 

An  example  of  a  different  kind  will  now  be  given  to  show 
more  clearly  a  genetic  sequence  in  forms.  Among  the 
Brachiopoda,  Atrypa  hystrix  represents  one  of  the  terminal 
members  or  species  of  a  line  of  varietal  and  specific  differ- 
entiation, extending  through  the  Silurian  and  Devonian.  The 
type  commonly  known  as  Atrypa  reticularis  appears  to  have 
had  its  inception  during  the  Ordovician ;  yet  in  the  Silurian 
it  is  found  as  a  conspicuous  and  fully  developed  form.  Here, 
also,  it  has  quite  a  wide  range  of  variation,  but  there  seems 
to  be  an  insensible  gradation  between  the  extremes,  which 
therefore  cannot  be  considered  as  definite  permanent  varie- 
ties. There  are,  however,  associated  forms  that  have  received 
distinctive  specific  names,  which  do  not  shade  into  each  other. 
During  the  early  and  middle  Devonian  certain  of  these  varia- 
tions in  the  main  stock  of  A.  reticularis  became  more  fixed, 
and  at  the  time  of  the  Hamilton  sediments  in  New  York, 
there  are  two  forms  known  as  A.  reticularis  and  A.  aspera, 
which  apparently  do  not  pass  into  each  other.  As  time  went 
on,  these  two  types  became  more  specialized  and  the  diver- 
gence correspondingly  increased,  until  in  the  Upper  Devonian, 
in  the  Chemung  sediments,  there  is  a  large  many-plicated 
A.  reticularis,  as  well  as  a  form  with  very  few  plications  and 
long  marginal  spines,  A.  hystrix.  Hall  and  Clarke31  thus 
summarize  the  stages  leading  to  the  formation  of  the  spinose 
forms:  "In  the  variant  of  Atrypa  reticularis,  occurring  in 
the  Niagara  fauna  at  Waldron,  Indiana,  the  free  concentric 
lamellae  frequently  show  a  tendency  to  fold  inward  at  the 
summit  of  the  principal  plications.  The  infolded  edges  fail 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  25 

to  unite,  and  this  tendency  to  the  formation  of  tubules  is 
apparently  carried  no  further  at  this  period.  More  extreme 
results  were  attained  by  the  Atrypa  aspera  of  the  Hamilton 
shales,  or  possibly  by  its  migrated  ancestor,  during  the  period 
of  time  represented  by  the  deposition  of  the  Lower  Helder- 
berg,  Oriskany,  and  Upper  Helderberg  sediments.  At  all 
events,  the  Atrypa  spinosa  of  the  Hamilton  shales  is  but  an 
A.  aspera  with  the  lamellae  enfolded  into  tubular  spines. 
Intermediate  stages  connecting  these  different  phases  are  not 
present  in  this  fauna.  .  .  .  This  spinose  form  is  continued 
into  the  Chemung  faunas  (A.  hystrix),  with  some  modifi- 
cation of  expression,  the  spines  being  few  and  long,  and  the 
plication  of  the  surface  very  coarse  and  quite  simple;  the 
shell  in  its  decline  thus  representing  a  decided  return  to 
the  primitive  type  of  structure."  H.  S.  Williams  72  has  classi- 
fied the  variations  in  the  stock  of  A.  reticularis  as  to  whether 
differentiation  in  the  number  of  plications  is  increased  or 
retarded,  and  concludes  that  the  extremes  are  most  strongly 
expressed  at  the  close  of  the  life-period  of  the  race.  The 
numerously  plicated  type  represents  the  accelerated  phase  of 
the  multiplication  of  radii,  while  A.  hystrix,  with  its  few  and 
coarse  radii,  represents  the  retardation  or  suppression  of  this 
tendency. 

The  only  great  group  of  animals  receiving  its  name  from 
its  characteristically  spinose  surface  is  the  Echinodermata,  or 
the  spiny-skinned  animals;  yet  it  is  extremely  doubtful 
whether  this  name  would  have  been  used  had  the  first  studies 
of  the  group  been  based  upon  the  Paleozoic  representatives, 
especially  the  pre-Devonian  species.  The  early  Sea-lilies 
(Crinoidea),  Cystideans  (Cystoidea),  Blastoids  (Blastoidea), 
and  Star-fishes  (Asteroidea)  had  smooth  or  nearly  smooth 
integuments.  In  its  early  genera,  even  the  most  typically 
spiny  class  of  the  whole  sub-kingdom,  the  Echinoidea  (Sea- 
urchins),  had  very  minute  and  insignificant  spines.  It  is  only 
in  the  late  Devonian  and  in  the  Carboniferous  that  truly 
spiny  forms  of  Crinoids,  Star-fishes,  and  Sea-urchins  are 
found. 


26  STUDIES  IN  EVOLUTION 

Of  equal  significance  is  the  fact  that  the  Echinodermata 
together  with  the  plants  represent  the  most  primitive  type  of 
structure,  one  in  which  there  is  a  more  or  less  circular 
arrangement  of  the  parts  or  organs.  The  Echinodermata 
are  the  highest  development  in  this  line  of  growth  among 
animals.  They  culminated  in  past  geological  ages,  and  from 
them  no  direct  line  of  descent  can  be  traced  (Bailey2  and 
Cope11). 

The  conclusion  from  the  study  of  the  phylogenies  of 
spinose  forms  is  parallel  to  the  one  drawn  from  the  ontogenies ; 
namely,  that  the  ancestors  of  spinose  as  well  as  non-spinose 
organisms  were  simple  and  inornate. 

CATEGORIES  OF  ORIGIN 

As  previously  shown,  spines  are  formed  either  by  growth 
or  by  suppression,  and  therefore  the  processes  determining 
their  production  are  either  constructive  through  concrescence 
or  destructive  through  decrescence.  Each  of  these  is  in  turn 
determined  by  forces  from  without  the  organism  (extrinsic) 
or  by  forces  from  within  (intrinsic).  In  this  connection  it 
is  of  no  especial  moment  whether  or  not  the  intrinsic  forces 
are  primary  or  are  an  immediate  or  subsequent  reflex  from 
the  extrinsic.  The  main  thing  is  the  direction  of  the  dom- 
inant force,  whether  centripetal  or  centrifugal.  If  in  some 
cases  it  can  be  shown  that  spine  development  has  been 
accomplished  by  intrinsic  forces  in  the  organism,  then  this 
development  may  be  brought  about  independently  of  the 
environment  and  possibly  at  variance  with  it.  Also,  if  in 
other  cases  the  extrinsic  forces  or  the  influences  of  the 
environment  have  caused  spine  growth,  it  may  in  some 
instances  illustrate  the  formation  and  transmission  of  an 
acquired  character,  or  at  least  the  operation  of  organic 
selection. 

The  point  has  now  been  reached  where  it  is  impracticable 
to  make  a  rigid  classification  of  the  direct  factors  or  an 
exact  determination  of  primary  and  secondary  causes.  It 


ORIGIN  AND   SIGNIFICANCE   OF  SPINES  27 

was  remarked  at  the  beginning  of  this  paper,  that  single 
causes  were  not  sufficient  in  every  case  to  account  for  spine 
growth,  and  while  it  is  comparatively  easy  to  formulate 
abstract  expressions  or  terms  covering  all  possible  cases,  it 
will  be  found  difficult  to  construe  properly  certain  factors  to 
fit  into  any  particular  conception.  In  illustration  of  this, 
the  foregoing  statements  may  be  taken.  Thus  spines  are 
formed  by  the  only  means  possible,  either  by  growth  of  new 
tissue  or  by  decrease  in  old.  Again,  the  forces  must  act 
from  the  interior  or  from  the  exterior ;  in  other  words,  they 
must  be  intrinsic  or  extrinsic.  But  in  some  specific  instance, 
while  considering  food,  forces  of  nutrition,  external  or  in- 
ternal demands,  reactions,  etc.,  a  question  may  arise  as  to 
the  proper  disposition  to  make  of  a  spine  developing  primarily 
by  external  stimuli  and  becoming  a  defence  and  secondarily 
a  weapon  ;  yet  which  by  differentiation  in  time  loses  some  of 
its  protective  and  offensive  qualities,  and  by  selection  may  be 
confined  to  one  sex. 

Growth  and  decline  are  underlain  by  the  processes  taking 
place  in  individual  cells  as  well  as  in  aggregates  of  cells,  for 
spine  growth  must  be  considered  in  unicellular  as  well  as 
multicellular  organisms. 

Ryder61  has  very  philosophically  discussed  the  correlations 
of  volumes  and  surfaces  of  organisms,  and  has  reached  the 
conclusion  that  "  the  physiological  function  of  a  cell  is  also  a 
function  of  its  figure,  i.  e.,  of  its  morphological  character ; 
that  is  to  say,  cells  tend  to  elongate  in  the  direction  of  the 
exercise  of  their  function."  Out  of  this  may  be  deduced  the 
correlative  conclusion  that  aggregates  of  cells  having  a  like 
function  also  tend  to  elongate  in  the  direction  of  the  exercise 
of  this  function ;  and,  further,  it  may  be  asserted  that  parts 
or  portions  of  cells  will  act  in  the  same  manner. 

A  familiar  illustration  of  these  principles  as  applied  to  a 
single  cell  may  be  taken  from  the  rhizopod  Amoeba  proteus. 
When  disturbed  by  incident  forces  in  all  directions,  it  assumes 
a  globular  form.  Under  continuous  motion  of  its  own,  it  is 
elongated  in  the  axis  of  motion,  its  larger  pseudopodia  being 


28  STUDIES  IN  EVOLUTION 

thrust  out  in  more  or  less  the  same  direction.  The  presence 
of  a  favorable  exciting  cause,  like  a  particle  of  food,  produces 
extension  of  the  protoplasm  to  envelop  it. 

Furthermore,  as  is  well  known,  continuous  extra-pressure 
on  any  part  of  an  organism  produces  atrophy  and  absorption, 
and  intermittent  or  occasional  pressure  causes  hypertrophy 
and  growth.  That  the  pressure  should  be  intermittent  seems 
a  necessary  condition  for  hypertrophy,  in  order  that  the 
parts  affected  may  have  normal  intervals  allowing  the  active 
exercise  of  nutrition.55  This  may  be  regarded  as  a  parallel 
statement  of  the  law  of  disuse  and  use ;  the  former  causing 
organs  or  parts  to  dwindle  away  and  lose  their  function,  and 
the  latter  producing  increased  nutrition  and  growth. 

This  ratio  of  exchange  between  nutrition  and  waste  is  on 
the  side  of  full  or  excessive  cell-nutrition,  producing  growth 
in  the  parts  affected,  while  deficiency  of  nutrition  produces 
decline  or  suppression.  If  the  successive  increment  constitut- 
ing growth  is  along  definite  progressive  lines  towards  higher 
structures,  and  the  decrement  affects  the  decline  of  useless 
parts  or  permits  of  the  replacement  of  a  lower  by  a  higher 
structure,  then  the  sum  of  the  changes  is  progressive 
evolution.* 

Growth,  as  stated,  seems  to  require  normal  intervals  for 
the  proper  exercise  of  nutrition,  which  involves  an  inter- 
mittence  of  the  exciting  or  stimulating  forces.  Rhythm  has 
been  shown  by  Spencer66  to  be  a  necessary  characteristic  of 
all  motion,  and  therefore  in  considering  either  the  intrinsic 
or  extrinsic  forces  acting  on  the  structures  of  an  organism, 
they  must  be  rhythmic  or  intermittent.  In  the  environment 
the  most  apparent  changes  are  those  of  light  and  darkness, 
heat  and  cold,  moisture  and  dryness,  and  variations  in  amount 
of  oxygen,  all  of  which  affect  an  organism  directly,  and  also 
through  the  accompanying  variations  in  the  character  and 
amount  of  the  food  supply,  the  number  of  enemies,  etc. 
These  and  most  of  the  mechanical  forces  of  the  environment 
are  therefore  intermittent,  and  their  resultant  must  have  a 

*  This  is  very  near  Cope's  idea  of  progressive  evolution. 


ORIGIN  AND   SIGNIFICANCE    OF  SPINES  29 

definite  tendency,  so  that  the  effects  are  not  with  each  change 
successively  positive  and  negative  to  the  same  degree ;  that 
is,  the  same  structures  or  adjustments  are  not  alternately 
made  and  unmade. 

It  is  generally  recognized  that  there  is  a  necessity  for  a 
force  or  energy  in  living  organisms,  which  is  not  the  imme- 
diate and  direct  result  of  external  agencies,  but  upon  which 
these  fall  and  produce  reactions.  It  is  considered  as  a  phase 
or  kind  of  vital  force  directing  growth,  and  therefore  a 
growth  force,  or  the  bathmic  force  of  Cope.10  The  internal 
energy  of  growth,  involving  the  capacity  or  effort  of  respond- 
ing to  external  stimuli,  is  termed  entergogenic  energy  by 
Hyatt.34  Without  this  power  an  organism  would  be  unable 
to  move  or  respond  to  external  stimuli.  The  effect  of  the 
action  of  this  kind  of  energy  must  be  the  resultant  between 
"the  structures  already  existent  in  the  organism  and  the 
external  forces  themselves."34  Since  the  growth  force  is 
within  the  organism,  or  inborn,,  it  is  one  of  the  principal 
characters  transmitted  through  heredity,  and  if  it  is  in  excess 
of  the  external  forces,  the  modifications  will  be  principally 
congenital  or  phylogenic.  If,  on  the  other  hand,  the  external 
forces  predominate,  the  modifications  will  be  principally  adap- 
tive, or  ontogenic.  In  each  case  the  resultant  is  the  actual 
visible  effect  of  the  two.  If  both  are  toward  the  establish- 
ment of  similar  structures,  their  effect  will  be  the  sum  of  the 
two;  but  if  they  are  opposed  to  each  other,  the  effect  will 
be  their  resultant,  the  nature  of  which,  as  seen  above,  will 
depend  upon  their  relative  power. 

These  conclusions  can  be  correlated  directly  with  the 
developmental  variations  occurring  in  the  life  history  of  any 
great  group  of  organisms.  Any  one  who  has  studied  the 
chronological  development  or  the  phylogeny  of  a  class  of 
forms  cannot  fail  to  have  been  impressed  with  the  fact  that 
all  types  of  life  are  physiologically  more  plastic  or  subject 
to  greater  changes  near  their  point  of  origin.  That  is,  the 
maximum  of  generic,  family,  and  ordinal  differentiation  is 


30  STUDIES  IN  EVOLUTION 

found  at  an  early  period,  while  the  greatest  specific  differen- 
tiation occurs  at  a  later  period.  This  shows  that  the  results 
of  variation  at  first  affect  the  physiological  and  internal 
structures,  and  that  later  the  changes  are  mainly  physical 
and  peripheral. 

One  explanation  of  this  would  be  that  the  forces  of  the 
environment  are  at  first  freely  transmitted  and  produce 
internal  modifications,  and  that  later  these  characters  become 
stable,  making  the  effects  of  the  external  stimuli  apparent  in 
the  superficial  differentiation  of  the  organisms. 

In  any  event  the  modifications  in  function  and  structure 
are  followed  by  modifications  in  surface,  showing  that  the 
more  important  physiological  and  structural  variations  are 
the  first  to  be  subjected  to  heredity  and  natural  selection, 
which  tend  to  fix  or  hold  them  in  check.  Features  of  less 
functional  importance,  as  peripheral  characters,  are  the  last 
to  be  controlled,  and  therefore  present  the  greatest  diversity, 
while  in  this  diversity  spinosity  is  the  limit  of  progress.  In 
order  to  be  hereditable,  the  modifications  through  the  environ- 
ment must  have  induced  correlative  internal  adjustments  and 
changed  forces  which  can  be  transmitted  to  offspring,  and 
they  in  turn  reproduce  the  specific  modifications. 

For  the  purpose  of  illustrating  these  statements,  the  evolu- 
tion of  the  Brachiopoda  and  Trilobita  will  be  taken.  The 
Brachiopoda  are  divided  into  four  orders,  all  of  which  appear 
in  the  Lower  Cambrian  and  continue  to  the  present  time. 
Schuchert64  states  that  "of  the  49  families  and  subfamilies 
constituting  the  class,  43  became  differentiated  in  the  Pale- 
ozoic, and  of  these  30  disappeared  with  it  ;"  also,  "of  the  327 
genera  now  in  use,  227  had  their  origin  in  Paleozoic  seas, 
or  nearly  70  per  cent  of  the  entire  class."  Throughout  the 
Cambrian,  "differentiation  was  mainly  of  family  importance." 
"Differentiation  is  most  rapid  near  the  base  of  the  older 
systems,  and  diminishes  the  force  from  the  older  to  the 
younger  geologic  divisions."  The  most  rapid  increase  was 
in  the  Ordovician,  the  culmination  was  in  the  Devonian,  and 
the  rapid  decline  came  with  the  Carboniferous.  About  six 


ORIGIN  AND   SIGNIFICANCE   OF  SPINES  31 

thousand  species  are  known,  and  of  these  probably  not  more 
than  one  hundred  and  fifty  are  living. 

Similar  data  are  derived  from  the  Trilobita.  This  group 
is  found  all  through  the  Paleozoic,  at  the  close  of  which  it 
became  extinct.  Two  of  the  three  orders  are  found  in  the 
Lower  Cambrian.  The  remaining  order  appeared  just  after 
the  close  of  the  Cambrian  in  the  early  Ordovician,  yet 
through  the  whole  of  the  remaining  sediments  not  a  single 
new  ordinal  type  was  developed.  When  applied  to  a  single 
order,  the  same  truth  comes  out.  The  order  Proparia  is 
one  whose  entire  history  can  be  traced,  extending  from  the 
Ordovician  through  the  Silurian  and  Devonian.  All  the 
families  appear  in  the  Ordovician ;  in  fact  not  a  single  family 
type  in  this  or  the  other  orders  was  produced  during  the 
whole  Silurian,  Devonian,  and  Carboniferous.6 

As  the  classes,  orders,  and  families  are  based  upon  the 
physiological  and  important  functional  structural  characters 
or  differences,  it  is  evident  that  at  or  near  the  beginnings  of 
their  life  history  is  found  the  demonstration  of  the  domina- 
tion of  phylogeiiic  over  ontogenic  characters. 

Conditions    or  Forces  affecting   Growth.  —  Since  spines    are 


purely  organic  structures,  their  production  must  follow 
general  laws  of  organic  change.  The  forces  considered  as  of  c 
most  consequence  are  two :  (1)  the  external  stimuli  from  the 
environment,  and  (2)  the  energy  of  growth  force.  These, 
with  their  opposites  (1  a)  the  restraint  of  the  environment, 
and  (2  a)  the  deficiency  of  growth  force,  are  believed  to 
include  the  chief  active  and  passive  causes,  not  only  of  spine 
production,  but  of  growth  and  decline  in  general.  Correlat- 
ing these  four  causes  with  their  constructive  and  destructive 
agencies,  together  with  their  extrinsic  and  intrinsic  modes  of 
action,  as  previously  explained,  there  result  (A)  the  external 
stimuli  of  the  environment  as  an  extrinsic  cause  of  concres- 
cence; (B)  energy  of  growth  force  as  an  intrinsic  cause  of 
concrescence;  (C)  external  restraint  as  an  extrinsic  cause 
of  decrescence ;  and  (D)  deficiency  of  energy  of  growth  force 
as  an  intrinsic  cause  of  decrescence.  The  remaining  vital 


32  STUDIES  IN  EVOLUTION 

forces  (nerve  force,  or  neurism,  and  thought  force,  or  phren- 
ism)  are  not  primary,  and,  although  doubtless  affecting 
growth  in  higher  organisms,  cannot  be  original  causes  appli- 
cable to  all  forms  of  life,  both  plant  and  animal. 

In  tabular  form,  the  divisions  and  relationships  of  the 
factors  of  spine  genesis  may  be  expressed  as  follows: 

A 

r  (  extrinsically      \  from  external 

/  (ceutripetally)  t  stimuli, 
'constructive  agencies  J 
(concrescence)  acting  "j  B 

I   (  intrinsically       /  from  growth 
*  (  (centrifugally)  J  force. 
Spines  originate  by^ 

C 

(  extrinsically      )  from  external 

,  1  (centripetafly)   i  restraint, 
destructive  agencies    I 
(decrescence)  acting    •[  D 

intrinsically      )  from  deficiency 
(centrifugally)  )  of  growth  force. 

Under  the  last  four  divisions  (A-D)  it  is  proposed  to 
discuss  the  origin  of  spines,  and  from  the  observations  made, 
to  derive  certain  conclusions  regarding  the  significance  of  the 
spinose  condition. 

A.  External  Stimuli. 

Under  external  stimuli  are  included  all  the  forces  of  the 
environment  (chemical,  physical,  organic,  and  inorganic) 
which,  through  their  impact  or  influence  on  an  organism, 
produce  a  consonant  favorable  change  or  disturbance.  In 
general,  it  will  be  seen  that  the  number  of  impressions  and 
their  power  will  depend  largely  upon  the  position  and  char- 
acter of  the  surface  upon  which  they  impinge.  The  more 
exposed  the  position,  the  greater  will  be  their  strength  and 
number,  and  if  these  stimuli  or  impressions  are  intermittent, 
and  not  so  violent  as  to  produce  waste  and  rupture,  growth 
will  ensue.  Under  ordinary  conditions,  exposed  parts  will 
naturally  be  the  first  to  receive  sufficient  stimulus  to  produce 
growth,  and  there  will  be  normally  a  direct  correlation  be- 
tween growth  and  stimulus.  In  a  simple  diagrammatic  form, 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  33 

this  would  be  expressed  by  a  series  of  lines,  the  first  repre- 
senting a  plane  surface.  Then,  owing  to  the  impossibility  of 
maintaining  a  uniformly  intermittent  stimulus  or  a  uniform 
response,  some  point  or  spot  on  this  surface  would  grow  in 
excess  of  the  others.  This  difference  would  be  augmented 
by  the  more  favorable  position  of  the  spot  to  receive  stimuli, 
further  growth  would  take  place,  the  growth  force  decreasing 
with  the  increase  of  distance,  and  the  final  action  of  these 
forces,  stimulus  and  growth,  would  be  to  produce  a  pointed 
elevation.  Such  structures  or  outgrowths,  especially  when 
made  of  hard  rigid  tissue,  would  be  termed  spines  under  the 
general  definition.  The  spine  may  be  viewed  as  an  attached 
organism,  and  its  conical  habit  of  growth  would  then  con- 
form to  the  law  of  radial  symmetry,  as  determined  by  the 
physiological  reaction  from  equal  radial  exposure  to  the 
environment.  That  all  the  irregularities  of  contour  in  all 
organisms  have  not  developed  into  pointed  processes  or 
spines  is  not,  therefore,  the  fault  of  the  simple  reciprocity 
between  growth  and  external  stimuli.  This  kind  of  develop- 
ment, however,  requires  a  direct  and  immediate  responsive 
external  growth  to  the  exciting  force,  which  from  various 
causes  is  frequently  absent.  Obviously,  stimuli  which  result 
simply  in  motion  or  equivalent  internal  adjustments  can  have 
no  effect  toward  spine  production,  so  that  only  the  results  of 
such  stimuli  as  bring  about  some  accompaniment  of  super- 
ficial growth  will  be  considered. 

With  the  exception  of  perfectly  spherical,  freely  moving 
forms,  all  organisms  have  certain  parts  which  are  more 
exposed  to  the  forces  of  the  environment  than  others,  and 
from  the  principles  already  enunciated,  such  exposed  parts 
under  normal  conditions  will  grow.  This  growth  in  the 
direction  of  function  and  stimulus,  when  acted  upon  by 
the  hereditary  functional  and  structural  requirements  of  the 
organism,  serves  to  produce  the  various  external  organs  and 
appendages.  But  when  the  surface  upon  which  the  stimuli 
fall  is  not  thus  predetermined  by  heredity  to  grow  into  a 
certain  organ  or  functional  part,  there  results  a  normal 

3 


34  STUDIES  IN  EVOLUTION 

responsive  action  between  growth  and  stimulus,  which,  as 
already  seen,  tends  to  produce  a  conical  or  spiniform  growth. 

Under  ordinary  favorable  conditions,  simple  external  stimuli 
acting  blindly  through  no  agencies  of  selection  would  develop 
spines  on  all  the  most  exposed  parts,  and  tend  to  differentiate 
ornamental  features.  This  has  been  the  case  with  many 
organisms  and  colonial  aggregates  possessing  no  power  of 
selection  or  not  acted  upon  by  any  forces  of  determination, 
conscious  or  unconscious.  In  such  cases  spines  may  or  may 
not  serve  for  protection,  and  their  function,  if  any,  can  be 
only  determined  separately  for  each  case.  If,  however,  the 
added  function  of  offence  is  included,  it  is  manifest  that  the 
spines  must  be  located  in  special  positions  adapted  to  use  for 
offensive  purposes,  as  on  the  tails  of  some  animals,  and  not 
necessarily  over  vulnerable  parts.  Here  the  selective  agency 
of  special  adaptation  is  shown.  Again,  if  while  there  is 
agreement  in  other  essential  characters,  spines  or  horns  are 
confined  to  either  sex,  it  is  evidently  a  case  of  sexual  selec- 
tion. Further,  if  they  develop  in  harmony  with  the  environ- 
ment, or  in  a  manner  parallel  to  similar  features  of  other 
organisms,  it  is  through  the  operation  of  physical  selection. 

Altogether,  under  the  general  forces  of  external  stimuli, 
there  are  five  aspects  in  which  to  consider  the  production  and 
growth  of  spines ;  namely, 

A.  From  External  Stimuli. 

A  1.  —  In  response  to  stimuli  from  the  environment  acting 
on  the  most  exposed  parts. 

A  2.  —  As  extreme  results  of  progressive  differentiation  of 
ornaments. 

A3.  —  Secondarily  as  a  means  of  defence  and  offence. 

A  4.  —  Secondarily  from  sexual  selection. 

A  5.  —  Secondarily  from  mimetic  influences. 

B.    G-rowtJi  Force. 

In  unicellular  organisms  growth  force,  or  bathmetic  energy, 
must  reside  wholly  in  the  germ  cell,  and  therefore  is  con- 
cerned with  reproduction  as  well  as  with  cell  differentiation. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  35 

In  multicellular  organisms  the  growth  force  is  in  both  germ 
and  soma  cells,  and  its  relative  strength  seems  to  depend 
upon  its  power  to  reproduce  lost  parts,  often  including  germ 
cells  as  well  as  soma  cells.  In  many  of  the  lower  classes 
the  growth  force  is  able  to  complete  a  structure  or  lost  part 
without  the  stimulus  of  use,  which  in  higher  animals  often 
seems  to  be  part  of  the  necessary  requirements  for  growth. 

Growth  itself  is  the  repetition  of  cells  under  nutrition  and 
stimulus,  and  the  latter  may  be  hereditary  or  extra-indi- 
vidual. It  is  now  recognized  that  since  the  division  of  a  cell 
makes  two  unlike  cells,  each  unlike  the  parent,  such  repeti- 
tion will  produce  structures  which  present  some  degree  of 
difference.  The  variation  is  therefore  a  necessary  quality 
of  growth,  and  its  degree  will  change  in  response  to  the 
differentiation  of  the  forces  affecting  growth. 

When  spines  which  have  arisen  from  intrinsic  growth  force 
only  are  sought,  it  is  apparent  that  they  cannot  be  distin- 
guished from  those  arising  from  external  stimuli  acting  on 
and  directing  the  growth  force,  unless  in  some  instances 
they  are  found  to  be  developed  independently  or  even  at 
variance  with  the  environment.  Because  spines  are  some- 
times useful  to  the  organism,  it  is  impossible  to  believe  that 
they  have  originated  from  that  cause,  since  their  existence  in 
some  form  must  precede  the  capacity  of  making  them  useful. 
After  they  began  to  develop  by  either  intrinsic  or  extrinsic 
forces,  their  being  found  useful  would  simply  tend  to  their 
conservation  and  further  development. 

Variation  which  is  not  restricted  by  natural  selection  or  a 
long  line  of  hereditary  tendencies  is  known  as  free  variation. 
It  is  best  exhibited  in  a  stock  which  occupies  for  a  consider- 
able time  a  region  favorable  in  respect  to  food,  climate,  and 
absence  of  dominating  natural  enemies.  This  relation  has 
been  called  the  period  of  "  Zoic  maxima "  by  Gratacap,23 
and  has  been  further  discussed  by  the  same  author,  under 
the  aspect  of  numerical  intensity.22  The  most  rapid  rise  of 
a  stock  is  considered  to  be  consequent  to  a  favorable  environ- 
ment and  high  vitality. 


36  STUDIES  IN  EVOLUTION 

In  illustration  of  these  points,  the  AchatinellsB  of  the  Sand- 
wich Islands  afford  a  good  example.  The  great  number  of 
species  on  these  islands  has  probably  been  evolved  since 
Tertiary  times,  and  the  process  of  specific  delimitation  is 
apparently  still  going  on,  for  species  are  now  to  be  found 
which  did  not  exist  fifty  years  ago  (Verrill);  also,  a  few 
species  formerly  common  are  now  obsolescent  or  extinct. 
According  to  Hyatt,  they  all  can  be  deducible  from  a  single 
species  which  has  differentiated  in  time  through  divergence, 
dispersion,  and  colonial  isolation.  In  early  times  birds  may 
have  fed  upon  them,  but  the  complete  or  partial  extinction 
of  the  former  by  man  has  resulted  in  complete  immunity  for 
the  arboreal  Achatinellse,  and  it  is  now  common  to  find 
several  of  the  most  highly  colored  varieties  feeding  together 
on  the  same  leaf.  The  modern  importation  of  pigs,  sheep, 
and  mice  on  the  islands  has  introduced  an  enemy  to  the 
terrestrial  species,  the  effects  of  which  are  already  being 
noticed.  In  specific  differentiation  and  in  individual  varia- 
tion, both  Hyatt  and  Verrill  regard  the  extraordinary  develop- 
ment of  this  type  as  characteristic  of  free  variation,  under 
favorable  conditions,  in  a  plastic  stock  which  has  not  yet 
reached  its  limits  nor  become  fixed. 

Among  the  Crustacea,  the  remarkable  evolution  of  the 
genus  Grammarus  in  Lake  Baikal,17  and  of  Allorchestes  in 
Lake  Titicaca,19  seem  to  furnish  parallel  examples.  Allor- 
chestes ranges  from  Maine  to  Oregon  and  southward,  through 
the  United  States,  Mexico,  and  South  America,  to  the  Straits 
of  Magellan.  Before  Lake  Titicaca  was  explored,  but  one 
or  two  authentic  freshwater  species  were  known  from  both 
continents;  yet  from  this  lake  basin  alone,  Faxon19  has 
described  seven  distinct  species,  constituting  the  entire 
crustacean  fauna  with  the  exception  of  a  species  of  Cypris. 
Several  species  are  "remarkable  among  the  Orchestidse  for 
their  abnormally  developed  epimeral  and  tergal  spines." 
These  and  the  species  of  G-ammarus  from  Lake  Baikal  will 
be  referred  to  again  later  in  this  paper.  It  is  simply  desired 
here  to  indicate  that  these  variations  in  Achatinella  and 


ORIGIN  AND  SIGNIFICANCE  OF  SPINES  37 

Allorchestes  have  arisen  from  a  single  parent  stock,  within  a 
small  geographic  province.  The  natural  interpretation  seems 
to  be  (a)  that  the  environment  is  favorable,  as  evinced  from 
the  great  number  of  individuals;  (£>)  that  this  has  favored 
and  increased  the  growth  force ;  and  (c)  that,  finally,  the  law 
of  multiplication  of  effects,  reproductive  divergence,67  the 
survival  of  the  unlike,  and  the  conservative  forces  of  natural 
selection  and  heredity  have  directed  the  growth  force,  and 
produced  the  specific  differentiation  which  is  now  found. 

A  factor  of  Evolution,  called  "  Reproductive  Divergence  " 
by  Vernon,67  seems  to  be  operative  here,  since  it  affords  an 
explanation  for  a  means  of  differentiation  in  a  single  stock 
under    a    common   environment.      As    this    factor    has   but 
recently  been  discussed,  it  may  well  be  defined  at  this  time, 
so  as  to  enable  a  direct  application  to  be  made.     Reprod 
tive  divergence  assumes  that  in  many  species  there  will  b( 
greater  fertility  between  individuals  similar  in  color,  form,  or£/  &/L 
size,  than  between  individuals  not  agreeing  in  these  respects,  /(  ^^ 
and    that  in    subsequent    generations    the    divergence   will  ./^ 
become  progressively  greater  in  respect  to  the  characteristic   y% 
in  question,  so  that  finally  the  original  stock  will  become^'    *-'' 
separated  into  distinct  varieties,  sub-species,  or  species. 

When,  from  any  cause,  the  forces  of  nutrition  are  directed 
toward  spine  production,  and  when  the  direct  results  are  .  ./ 
accomplished  in  the  reciprocal  formation  of  one  or  more 
spines,  there  is  often  an  apparent  inductive  influence  or 
impulse  given  to  growth  toward  the  further  production  or 
repetition  of  spines.  This  may  result  in  the  formation  of 
compound  spines,  or  a  group  of  spines,  or  even  produce  a 
generally  spinous  condition. 

Naturally,  spines  arising  through  growth  force  may  be 
useful  for  defence  and  offence,  and  the  selective  influences 
of  sex  and  mimicry  may  also  tend  to  greater  development 
and  elaboration.  Furthermore,  growth  forces  reacting  on 
any  external  structures,  as  lines,  lamella,  ribs,  nodes,  etc., 
may  tend  to  differentiate  such  ornaments  into  spines. 

Therefore,  under  the  general  consideration  of  spines  pro- 


38  STUDIES  IN  EVOLUTION 

duced  through  growth  force,  the  following  factors  are  offered 
for  consideration: 

B.  From  Growth  Force. 

B  1.  — Prolonged  development  under  conditions  favorable 
for  multiplication. 

B  2.  —  By  repetition. 

B  3.  —  Progressive  differentiation  of  previous  structures. 

B  4.  —  Secondary  development  through  the  selective  influ- 
ences of  defence,  offence,  sex,  mimicry,  and  other  external 
demands. 

C.  External  Restraint. 

Intermittent  stimulus,  as  previously  shown,  produces 
growth  in  the  direction  of  function.  When  the  growth 
equals  the  waste,  an  equilibrium  or  static  condition  is 
reached,  and  no  relative  change  occurs.  The  absence  of 
either  extrinsic  or  intrinsic  stimulus  will  not  be  favorable 
to  growth,  and  under  such  conditions  an  organ  or  struc- 
ture may  remain  undeveloped,  or,  if  already  present  in  the 
organism,  it  may  waste  away  and  degenerate  into  a  vestigial 
structure,  or  even  disappear  altogether. 

On  the  other  hand,  it  is  well  known  that  continuous  pres- 
sure not  only  prevents  growth,  but  in  addition  resorption 
takes  place,  and  in  this  way  the  whole  or  a  portion  of  a 
structure  may  be  removed.  These  changes  have  frequently 
been  studied  in  embryos,  as  well  as  in  many  internal  struc- 
tures, and  are  also  familiar  in  the  enlarged  pedicle-openings 
of  many  Brachiopoda,  caused  by  pressure  of  the  pedicle, 
and  in  the  similar  opening  for  the  byssal  plug  of  Anomia. 
Packard 54  gives  examples  among  the  Crustacea  and  Insecta, 
which  are  clearly  to  the  point.  He  says  of  the  Crustacea, 
"It  may  here  be  noted  that  the  results  of  the  hypertrophy 
and  overgrowth  of  the  two  consolidated  tergites  of  the  second 
antennal  and  mandibular  segments  of  the  Decapod  Crustacea, 
by  which  the  carapace  has  been  produced,  has  resulted  in  a 
constant  pressure  on  the  dorsal  arches  of  the  succeeding  five 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  39 

cephalic  and  five  thoracic  segments,  until  as  a  result  we  have 
an  atrophy  of  the  dorsal  arches  of  as  many  as  ten  segments, 
these  being  covered  by  the  carapace." 

The  restraint  of  the  environment  through  unfavorable  con- 
ditions is  the  antithesis  of  A,  or  the  influence  of  constructive 
external  stimuli,  and  is  considered  as  the  extrinsic  operation 
of  destructive  agencies.  It  is  evident  that  external  unfavor- 
able conditions  will  repress  growth,  with  a  resultant  atrophy 
of  the  structures  affected.  In  this  way,  also,  the  environment 
may  cause  the  disuse  of  an  organ,  which  by  consequent  sup- 
pression may  dwindle  away  to  a  spine,  as  in  the  leaves  and 
branches  of  desert  plants,  and  the  spurs  of  the  Python 60 
representing  the  hind  limbs.  It  may  likewise  repress  growth, 
as  in  the  spines  on  the  lower  side  of  the  poriferous  coral 
Michelinia  favosa,^  representing  aborted  attempts  at  bud- 
ding, the  failure  being  due  to  the  unfavorable  position  of  the 
buds  for  securing  food. 

The  restraint  of  the  environment  may  also  act  in  a  mechan- 
ical manner  to  produce  spines,  as  will  be  shown  subsequently 
in  some  Brachiopoda  and  Trilobita.  Furthermore,  spines 
arising  through  any  phase  of  external  restraint,  may  second- 
arily come  under  the  influences  of  natural  selection,  and  be 
useful  for  protection  and  offence,  or  conform  to  other  external 
demands. 

Under  the  head  of  external  restraint,  therefore,  are  the 
following  categories: 

C.  From  External  Restraint. 

C  1.  —  Restraint  of  environment  causing  suppression  of 
structures. 

C  2.  —  Mechanical  restraint. 

C  3.  —  Disuse. 

C  4.  —  Secondarily  for  protection,  offence,  etc. 

D.   Deficiency  of  Growth  Force. 

The  growth  force  in  organisms  may  be  reduced  in  several 
ways,  the  most  general  and  obvious  modes  being  by  an 


u- 

t    <f  rf^ing 
• 


40  STUDIES  IN  EVOLUTION 

unfavorable   environment,    lack   of  physiological   plasticity, 
too  close  interbreeding,  pathologic  influences,  and  parasitism. 
The  first  commonly  implies  a  scarcity  of  food,  or  it  may  be 
that   the   temperature,   moisture,   light,    elevation,   or   other 
conditions  are  unsuitable  to  the  normal  development.     The 
lack  of  physiological   plasticity  affects  growth  force  by  its 
resistance  to  change,  and  is  most  strongly  apparent  in  highly 
forms.    The  effects  of  close  interbreeding  in  reduc- 
vitality  are  too  well  known  to  require  further  notice. 
Only  in  exceptional  instances   can   individual   pathologic 
(  Conditions   have  any  effect  on  a  stock.     The  retrogressive 
.  t/V^-  f£!$eries  of  animals  which  are  diseased  in  appearance,  and  are 
jjC^^lf.    considered  by  Hyatt  ®*  as  akin  to  pathologic  distortions,  are 
apparently  types  which  have  ceased  to  advance  physiologi- 
cally, and  are  therefore  only  adapted  to  special  sets  of  con- 
ditions.    In  these,  the  pathologic  or  abnormal  condition  is 
racial   instead   of  individual,  and   its  cause  seems  to  be  a 
deficiency  of  vital  power  combined  with  great  external  differ- 
entiation, the  final  result  being  the  assumption  of  characters 
belonging  to  second  childhood  and  ending  in  extreme  senility, 
with  the  loss  of  spines  and  other  ornaments. 

The  life  history  of  parasitic  organisms  shows  their  origin 
from  higher  normal  types  by  a  process  of  retrogression  through 
loss  of  motion  and  disuse  of  parts.  Their  mode  of  living 
implies  dependence  upon  the  vitality  of  an  immediate  host, 
and  altogether  they  may  be  deficient  in  the  energy  of  growth 
force. 

Any  of  the  preceding  factors,  single  or  combined,  acting 
upon  an  organism  or  group  of  organisms  will  produce  sup- 
pression of  structures  or  functions.  Whether  from  external 
or  internal  causes,  the  waning  and  disappearance  of  characters 
are  almost  always  inversely  to  the  order  of  their  development 
or  appearance,  either  in  the  race  or  in  the  individual,  and 
the  most  primitive  or  axial  characters  are  therefore  the  most 
persistent  and  the  last  to  disappear.  In  this  way  a  leaf  may 
be  suppressed  into  a  spine  representing  the  midrib,  a  branch 
into  a  spiniform  twig,  a  leg  or  digit  into  a  spine,  etc.  As 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  41 

in  other  primary  causes  of  spine  genesis,  there  may  also  come 
secondary  influences  of  protection,  offence,  etc.,  controlled  by 
natural  selection. 

It  will  be  convenient  to  consider  spine  production  from  lack 
of  growth  force  under  three  heads : 

D.  Deficiency  of  Growth  Force. 

D  1.  —  Intrinsic  suppression  of  structures  and  functions. 

D  2.  —  Disuse. 

D  3.  —  Secondarily  for  protection,  etc. 

Summary  of  Causes  of  Spine  G-enesis. 

Before  taking  up  in  more  detail  the  various  causes  of  spine 
development,  and  illustrating  them  by  means  of  examples 
drawn  from  a  number  of  classes  of  organisms,  it  is  well  to 
restate  the  factors  which  are  believed  to  induce  spine  growth. 
This  is  especially  desirable  from  the  fact  that,  through  the 
operation  of  unlike  forces,  similar  conditions  may  produce 
the  same  morphological  results,  as  in  the  differentiation  of 
ornamental  lamellse  and  ridges,  which,  either  from  external 
stimuli  or  dispersion  of  growth  force,  may  develop  into 
spines.  In  such  cases  it  is  difficult  or  impossible  to  distin- 
guish the  primary  force,  and  the  only  satisfactory  method  is 
to  discuss  the  subject  under  one  head. 

By  carrying  out  this  plan,  and  indicating  the  instances 
where  the  causes  may  replace  or  overlap  each  other,  it  may 
be  shown  how  spines  have  originated,  as  follows :  — 

I.  In  response  to  stimuli  from  environment  acting  on  most 
exposed  parts.     (Ai.) 

II.  As  extreme  results  of  progressive   differentiation   of 
previous  structures.     (A2,  B3.) 

III.  Secondarily,  as  a  means  of  protection  and   offence. 
(A3,  B4,  C4,  D3.) 

IV.  Secondarily  from  sexual  selection.     (A4,  B4,  C4,  D3.) 

V.  Secondarily  from  mimetic  influences.    (A5,  B4,  C4,  D3.) 


42  STUDIES  IN  EVOLUTION 

VI.  Prolonged  development  under  conditions  favorable  for 
multiplication.     (Bi.) 

VII.  By  repetition.     (B2.) 

VIII.  Restraint   of  environment   causing   suppression   of 
structures.     (Ci.) 

IX.  Mechanical  restraint.     (C2.) 

X.  Disuse.     (C3,  D2.) 

XL    Intrinsic  suppression  of  structures  and  functions.  (Di.) 

To  illustrate  the  various  causes  of  spine  growth,  represent- 
ative examples  which  are  believed  to  conform  to  the  require- 
ments will  be  selected  from  various  groups  of  organisms. 
The  number  of  spinose  forms  is  so  great  that  it  will  be  impos- 
sible to  give  more  than  the  briefest  citation  of  a  few  of  the 
leading  types,  especially  those  which  have  come  under  the 
notice  of  the  writer;  on  this  account  the  number  of  examples 
derived  from  the  vegetable  kingdom  will  be  necessarily  few. 

I.  In  response  to  stimuli  from  the  environment  acting  on 
most  exposed  parts.     (A^) 

The  action  of  external  stimuli  falling  on  the  most  exposed 
parts  of  organisms  is  probably  one  of  the  most  fundamental 
and  fertile  causes  of  spine  production,  since  the  relation 
between  cause  and  effect  is  more  direct  and  apparent  here 
than  by  other  modes  of  origin.  In  a  general  way  it  com- 
prehends all  the  remaining  causes  coming  under  the  head 
of  external  stimuli,  but  for  present  purposes  it  will  be 
restricted  by  the  elimination  of  secondary  conditions,  such 
as  the  indirect  production  of  spines  through  differentiation 
of  previous  structures,  and  the  action  of  external  forces  of 
selection. 

The  ruling  forces  in  plants  being  so  largely  vegetative,  or 
those  of  growth,  and  the  cause  of  variation  being  principally 
physico-chemical  and  not  molar,  most  of  the  modifications 
to  produce  spines  will  fall  under  other  categories  of  origin 
(B,  D)  than  the  one  now  under  discussion. 

In  the  free  swimming  forms,  however,  as  the  desmids  and 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  43 

diatoms,  the  external  relations  are  found  to  be  very  much 
like  those  of  animals.  The  frustrule  of  the  diatom  Attheya 
decora  47  is  quadrate  in  outline,  and  from  the  angles  there 
extend  sharp  spinous  processes,  as  represented  in  figure  32. 
The  frustrule  of  the  desmid  Staurastrum  cuspidatum  is  com- 
posed of  two  triangular  halves,  and  the  spines  project  from 
the  vertices  of  the  angles.  Other  species  of  Staurastrum, 
Xanthidium  (X.  armatum  47),  Arthrodesmus  (A.  octocornis 59), 
etc.,  show  similar  spine  growth  from  the  most  prominent 
portions  of  the  frustrules.  It  is  evident  that  in  forms  like 
these  having  angular  outlines,  any  growth  produced  by  exter- 
nal stimuli  will  naturally  be  greatest  at  the  points  of  these 
angles,  and  in  conformity  with  the  previous  analyses  of  these 
factors  a  spiniform  extension  of  the  tissues  would  result. 

Among  the  freshwater  Rhizopoda  belonging  to  the  Pro- 
toplasta  (=  Amoelina),  the  genus  Difflugia  affords  good 
examples.  D.  globulosa*1  has  a  nearly  spherical  shell.  In 
D.  pyriformis  41  the  shell  is  elongate  pear-shaped,  and  gener- 
ally round  on  the  summit  or  fundus,  though  in  rare  instances 
a  central  spiniform  elevation  is  developed.  This  tendency 
becomes  fixed  in  D.  acuminata41  in  which  the  shell  in  general 
form  resembles  the  preceding  species,  but  the  fundus  is 
commonly  prolonged  into  a  single  acuminate  process  (figure 
33),  though  occasionally  two  or  three  spines  are  found.  In 
D.  corona  41  there  is  a  circlet  of  spines  around  the  margin  of 
the  fundus  besides  the  primary  one  in  the  centre.  Difflugia 
constricta  41  is  a  variable  form,  with  the  top  of  the  shell  gen- 
erally smooth,  though  sometimes  it  is  acuminate,  and  occa- 
sionally it  has  two  or  even  a  cluster  of  spines  (figures  34-36). 
Euglypha  mucronata41  has  a  terminal  spine,  as  in  Difflugia 
acuminate.*1  Placocista  spinosa*1  is  a  flattened  mitre-shaped 
form  with  a  distinct  edge,  along  which  are  numerous  spines. 
It  should  be  noted  that  no  spines  are  developed  on  any 
portion  of  these  freshwater  Rhizopoda  except  those  here 
mentioned. 

The  Nasellarian  Radiolaria  furnish  many  instances  of  a 
terminal  spine  from  the  summit  of  the  silicious  helmet  or 


44 


STUDIES  IN  EVOLUTION 


33 


36 


cup-shaped  skeleton;  as  in  Eucyrtidium  elegans?*  Podocyrtis 
Schomburgki^  Tridictyopus  conicus^  Cornutella  hexagona^ 
etc.  Many  of  the  primary,  or  axial,  spines  in  other  sub- 
orders probably  originated  according  to  I.  In  the  Spumella- 
32  34  rian  forms  especially,  the  princi- 

vx  x^88k    Pa^  spines  project  from  the  prom- 

[Jj  inent  portions;  as  in  Trigonactura 

^jjjjjr  triacantha™  Hymenacturacoper- 
nici?Q  Rhopalastrum  triceros,^  R. 
hexaceros,2Q  etc.  The  existence  of 
similar  non-spinose  species  shows 
that  the  formation  of  spines  is  in- 
dependent of  the  growth  of  the 
normal  prominences;  as  in  Rho- 
palastrum malleus^  R.  hexago- 
num^Q  etc. 

In   the    Foraminifera    the    con- 
figuration of  certain  forms  is  such 
that   parts    of  the    test  are  much 
FIGURE  32.  -  Attheya  decora,  more  prominent  than  others,  and 
a  diatom,  with  spines  from  the  in    these    exposed    situations    the 

angles.- (From  Mic^Dict.)  {  t  f  ntl      d        ^ 

FIGURE  33.  —  D(fflugia  acumi-      *  J 

nata,     a     freshwater     rhizopod;    Oped.        Some     of     the     triangular 

showing  spiniform  projection  of  Textularia3  have  spines  at  the  two 

thefundus.  X  200.  (After  Leidy.)    . 

FIGUBE    M.  —  D!fflugia  con-  lateral    angles   on   the    oral   side. 

stricta,  a  freshwater  rhizopod,  Some  of  the  individuals  of  Textu- 
Avith  rounded  fundus.  X  175.  7  .  /.  7.  Q  i  ,->  •  -i 

(After  Leid  )  IcLTia  jolium y    show    that  similar 

FIGURE     35.  —  The     same ;  spines  were  developed  at  different 

showing  a  single  spine   on   the      t  f  growth,  SO  that,  in  a  full- 

fundus.  X  175.     (After  Leidy.)  6  5  . 

FIGURE  36.    The  same;  show-  grown  specimen,  there  may  be  two 
ing  two  spines,    x  175.    (After  or  three  pairs  of  spines  along  the 

sides.       Others,    like    Verneuilina 

spinulosa*  and  Colivina  pygmoea^  develop  spines  from  the 
points  of  each  chamber.  A  number  of  species,  also,  show  a 
single  spine  at  the  apex  of  the  shell ;  as  Pleurostomella  alter- 
nans,g  Bolivina  robusta^  Polymorphina  sororia,  var.  c-uspi- 
data,*  etc.  In  the  latter  species  the  ordinary  form  is  rounded 
or  obtusely  pointed  at  the  fundus. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  45 

Some  of  the  Infusoria  have  terminal  spiniform  processes, 
which,  by  analogy  with  other  forms,  have  probably  developed 
according  to  I;  as  Ceratium  tripos?  0.  longicorne,*  C.fusus.9 

The  apertural  spines  on  some  of  the  graptolites  are  on  the 
most  exposed  portions  of  the  hydrotheca;  as  in  Monograptus 
spinigerusf2  Dicranograptus  Nidiolsonif*  Retiograptus  tenta- 
culatus,  and  Graptolithus  quadrimucronatus.  In  many  com- 
pound corals  the  corallites  are  polygonal  from  crowding,  and 
the  most  exposed  portions,  the  angles  of  the  calices,  often 
bear  spines;  as  Favosites  spinigerus^  Callopora  exsul,®  etc. 
The  spines  on  the  septa  and  costse  of  corals  probably  originate 
by  intrinsic  forces  (B),  since  they  are  internal  growths  not 
influenced  directly  by  external  stimuli. 

The  spines  on  the  ventral  sacs  of  Crinoidea  are  usually 
terminal,  and  in  the  most  exposed  situations;  as  in  Scy- 
talocrinus  validus,^  Dorycrinus  unicornis,®  Aulocrinus  Agas- 
sizi^  etc. 

The  anterior  and  posterior  pairs  or  rows  of  spines  on  the 
loricse  of  some  species  of  Rotator^a  are  in  the  most  exposed 
places;  as  in  Anurcea  squamula,  Noteus  quadricornis,  etc. 
The  spinules  on  the  tubes  of  Spirorbis  are  usually  developed 
after  it  rises  above  the  object  of  support  so  as  to  be  exposed 
on  all  sides;  as  Spirorbis  spinuliferus.51 

The  spinules  at  the  corners  of  the  angular  cell  apertures 
of  many  Bryozoa  are  in  the  most  exposed  situations,  and 
probably  arise  through  external  stimuli;  as  in  Trematopora 
echinata,®*  T.  spiculata,^  etc.  The  large  marginal  spines  of 
the  brachiopod  Atrypa  hystrix  31  probably  owe  their  excessive 
development  to  external  stimuli,  though  the  phylogeny  of 
the  species  shows  that  the  spines  first  originated  through  the 
differentiation  of  the  radiate  and  concentric  ornaments. 

In  many  pelecypods  the  siphonal  region  receives  a  great 
amount  of  stimulus,  and  the  post  umbonal  slope  is  the  part 
most  exposed.  Along  this  slope  are  found  many  of  the 
spines,  and  generally  the  greatest  differentiation  of  ornament. 
Examples  of  spines  on  post-umbonal  slopes  may  be  seen  in 
Oallista  sublamellosa  and  young  Saxicava  arctica  (figure  27). 


46  STUDIES  IN  EVOLUTION 

Such  spines  represent  periodic  extensions  of  the  mantle 
border,  and  in  some  cases  the  stimulus  for  this  growth  may 
come  from  internal  causes.  The  spines  on  Unio  spinosus  and 
related  species  are  believed  by  Mr.  Charles  T.  Simpson  to 
assist  in  anchoring  the  shell  in  the  sand  of  swift  running 
streams.  In  Callista,  the  young  Saxicava,  and  the  Unios 
mentioned,  the  spines  occur  on  all  individuals  and  at  such  an 
early  period  as  to  preclude  any  special  sexual  function. 

In  the  Gastropoda  the  periodic  extension  of  the  shell  over 
the  posterior  canal  and  the  spiniform  prominences  formed 
on  the  labrum  are  situated  in  exposed  places,  or  where  the 
amount  of  stimulus  is  greatest;  as  in  Trophon  magellanicus, 
Strombus  pugilis,  Fusus  coins,  Clavatula  mitra,  Melo  dia- 
dema,  etc. 

The  spines  on  the  larvae  of  geometrid  moths  are  usually  on 
top  of  the  loop,  and  are  explained  by  Packard54  as  follows: 
"  The  humps  or  horns  arise  from  the  most  prominent  portions 
of  the  body,  at  the  point  where  the  body  is  most  exposed  to 
external  stimuli." 

When  the  origin  and  function  of  spines  in  a  great  many 
forms  of  animals,  and  especially  among  the  higher  classes, 
are  examined,  it  seems  almost  impossible  to  decide  whether  a 
spine  has  been  originated  and  perpetuated  by  free  variation 
and  heredity,  or  by  the  general  action  of  external  stimuli 
on  the  most  exposed  parts;  and  in  the  latter  case,  whether 
or  not  under  the  selective  influences  of  use.  Its  origin  in 
either  instance  may  be  through  external  stimuli,  but  in  the 
latter,  it  falls  under  other  captions  than  AA;  or,  in  other 
words,  the  external  stimuli  excite  the  growth  force  at  certain 
points,  and  the  growths  so  produced  may  be  simply  reciprocal 
without  function  or  they  may  serve  purposes  of  protection  or 
offence.  Thus  the  dorsal  and  rostral  spines  on  the  zoea  of 
the  Decapoda  are  on  the  most  exposed  points,  and  seem 
to  function  as  defensive  structures.  As  soon  as  the  legs 
become  well  developed  or  when  the  animal  ceases  to  swim  at 
the  surface  and  hides  among  the  stones,  etc.,  at  the  bottom, 
these  spines  become  reduced  and  are  often  succeeded  by 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  47 

others.  The  spines  of  the  adult  are  also  usually  efficient  for 
protection,  but  owing  to  the  change  in  form  of  the  animal 
and  change  of  habitat,  the  most  exposed  parts  are  different 
from  those  of  the  larva,  and  the  spines  are  frequently 
developed  where  there  were  no  larval  spines;  as  in  Cancer 
irroratus,  Callinectes  hastatus,  etc.  Again,  the  horned  ungu- 
lates show  in  their  habits  of  sport,  fighting,  defence,  and 
procuration  of  food,  that  the  exposed  angles  of  the  top  of  the 
skull  are  subject  to  the  greatest  number  of  stimuli,  and  there 
the  horns  are  developed.  The  connection  between  external 
stimuli  and  growth  is  here  most  manifest,  for  it  is  impossible 
to  imagine  the  action  of  free  variation  or  simple  growth  force 
as  resulting  independently,  in  the  evolution  of  horned  out 
of  hornless  species  in  several  sub-orders  of  mammals,  and 
in  every  case  determining  the  location  of  the  horns  on 
the  prominent  angles  of  the  skull,  whether  on  the  nasals,  ; 
maxillaries,  f rentals,  or  parietals. 

It  is  well  known  that  toads  and  frogs  defend  themselves 
by  using  the  head  as  a  shield,  and  the  cranial  angles  thus 
receive  the  greatest  amount  of  stimulus.  "  There  are  natural 
series  of  genera  measured  by  the  degree  of  ossification  of  the 
superior  cranial  walls  "  (Cope 10).  In  the  highest  genera  the 
head  is  completely  encased,  and  in  some  forms  the  project- 
ing angles  are  developed  into  short  horns.  The  so-called 
"  Horned  Toad  "  (Phrynosoma)  has  the  same  habit  of  defence, 
and  it  is  believed  that  this  mode  of  protection  or  of  receiving 
impacts  has  given  rise  to  the  structure,  by  stimulating  growth 
at  these  points. 


II.    As  extreme  results  of  progressive  differentiation  of 
previous  structures.     (A2,  B3.) 

The  differentiation  of  existing  ornamental  structures  into 
spines  has  already  been  noticed  in  several  instances  in  this 
article.  It  was  shown  that  spines  often  arise  by  the  elonga- 
tion of  nodes  and  tubercles  or  similar  structures,  by  rhythmic 
alternating  areas  of  growth  in  lamellae  and  ridges,  and  by 


48  STUDIES  IN  EVOLUTION 

the  growth  of  matter  at  the  intersections  of  lines,  lamellae, 
ridges,  etc.  Furthermore,  it  was  indicated  that  this  progres- 
sive differentiation  could  be  produced  either  (a)  by  the  direct 
action  of  external  stimuli  affecting  the  amount  of  nutrition 
brought  to  a  certain  structure,  (6)  by  the  stimulus  and  dis- 
persion of  growth  force,  or  (c)  by  a  combination  of  the  two 
forces.  In  this  differentiation  of  the  features  which  are 
generally  called  "ornamental,"  it  will  also  be  shown  that  the 
spine  is  the  final  result  of  progressive  differentiation,  and,  as 

?  previously  indicated,  can  be  formed  out  of  a  variety  of  other 
structures.  The  term  "  ornamental  "  is  mainly  one  of  human 
interpretation,  and  is  used  simply  in  apposition  to  "plain" 
or  "  simple ;  "  for  example,  a  clam  cannot  be  imagined  as 
consciously  favoring  a  particular  kind  or  arrangement  of 
tubercles  for  ornamental  purposes. 

In  a  reticulate  or  cancellate  surface  formed  by  the  crossing 
of  raised  lines,  ridges,  or  lamellae,  it  is  evident  that  the 
causes  or  forces  producing  such  structures  will  be  increased 
at  the  points  of  intersection,  and  normally  the  amount  of 
growth  will  here  be  greatest.  In  this  way  it  is  possible 
to  account  for  the  very  common  presence  of  spines  at  the 
intersections  of  the  radiating  and  concentric  lines  on  many 
Mollusca  and  other  organisms. 

A  few  examples  will  now  be  given  illustrating  the  differ- 
entiation of  various  structures  into  spines. 

The  points  of  intersections  of  the  elements  of  the  lattice  in 
the  Radiolaria  are  where  spines  are  most  frequently  found ;  as 
in  Larnacalpes  lentellipsis,  Orosphcera  Huxleyi,  Carposphcera 
melitomma,  etc.26  In  Xiphosphcera  pallas,  the  ridges  about 
the  openings  or  meshes  are  granular,  and  the  intersections 
are  raised  into  spines.  Many  of  the  discoid  shells  have  their 
edges  differentiated  into  spines;  as  Heliodiscus  asteriscus, 
H.  cingilluni)  H.  glyphodon,  Sethastylus  dentatus,  Heliodry- 
mus  dendrocydus,  etc.  When  an  edge  becomes  elevated  and 
defined  as  a  carina,  this  structure  is  also  often  spiniferous ;  as 
in  Tripocalpis  triserrata  and  Astropilium  elegans.  The  final 
differentiation  of  the  radiate  arrangement  in  the  Radiolaria 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


49 


results  in  forms  consisting  only  of  a  composite  spine ;  as  in 
the  legion  Acantharia. 

In  the  Foraminifera  there  are  many  instances  of  the 
gradual  differentiation  of  carinse,  ribs,  costse,  etc.,  into  spines. 
In  Bulimina  aculeata  9  the  surface  nodes  and  granules  become 
developed  into  spines.  In  Textularia  carinata9  and  Cristel- 
laria  calcarQ  the  carinae  are  spiniferous.  The  young  of 
Uvigerina  aculeata 9  is  strongly  costate,  and  later  shell  growth 
shows  the  costse  broken  up  into  numerous  spines.  A  re- 
lated species  ( U.  asperula 9)  has  the  whole  test  covered  with 
spinules,  which  are  sometimes  arranged  in  lines,  showing 
derivation  from  costse.  In  Truncatulina  reticulata9  the 
carina  is  made  up  of  confluent  spines,  often  discrete  along 
the  edge,  and  sometimes  entirely  separated. 


37 


38 


39 


FIGURE  37.  —  Cyaihophycus  reticulatus.     Ordovician.     £. 

FIGURE  38.  —  Dictyospongia  Conradi.     Devonian.     \. 

FIGURE  39.  —  Hydnoceras  tuberosum.  Devonian.  £.  (Figures  37,  38,  39, 
after  Hall.) 

To  illustrate  progressive  chronogenetic  and  ontogenetic  differentiation  in  a 
family  of  hexactinellid  sponges. 

The  hexactinellid  sponges  belonging  to  the  family  Dictyo- 
spongidse  show  some  very  clear  instances  of  the  progressive 
differentiation  of  ornament  in  time  and  in  ontogeny.  The 
Ordovician  Cyathophycus  reticulatus28  is  a  turbinate  form, 
with  a  rectangular  mesh  of  longitudinal  and  transverse 
spicular  rays  (figure  37).  At  more  or  less  regular  intervals 


50  STUDIES  IN  EVOLUTION 

some  of  the  spicules  are  larger,  thus  dividing  the  surface 
into  larger  rectangular  areas.  In  Dictyospongia  prismatica  28 
from  the  Devonian,  the  domination  of  eight  of  the  longi- 
tudinal bundles  of  spicules  has  produced  a  prismatic  form. 
D.  Oonradi  is  regularly  an  eight-sided  pyramid  or  prism 
when  young,  but  with  the  growth  and  elongation  of  the 
sponge  it  developed  slight  undulations,  then  nodes,  and 
later  prominent  tubercles  (figure  38).  Ceratodictya  annulata 
and  Hydnoceras  nodosum28  show  a  further  specialization  in 
the  formation  of  rings  and  nodes.  Practically  the  limit  to 
these  specializations  is  attained  in  Hydnoceras  tuberosum28 
(figure  39),  H.  phymatodes,  and  related  forms.  In  H.  tubero- 
sum  the  apex  representing  the  young  stage  or  the  initial 
growth  is  much  like  Cyathophycus  or  Dictyospongia.  This  is 
followed  by  a  prismatic  stage  like  D.  prismatica  and  D.  Con- 
radi-,  then  the  nodes  and  tubercles  are  introduced  and  further 
growth  produces  the  typical  characters  of  the  species.  The 
tubercles  are  surmounted  by  a  sharp  spine  formed  at  the 
intersection  of  two  spicular  laminse,  one  concentric  and  one 
longitudinal. 

Another  type  of  surface  specialization  is  shown  in  the 
genus  Physospongia  from  the  Keokuk  group  of  the  Lower 
Carboniferous.  In  this  genus  there  are  bands  of  regular, 
alternating,  elevated,  and  depressed  quadrules,  the  former 
frequently  having  the  superficial  layer  of  spicules  extended 
into  a  spiniform  process;  as  in  P.  Dawsoni.28 

Among  corals  there  is  occasionally  some  evidence  of  the 
external  differentiation  of  structures  into  spines.  The  epi- 
theca  of  the  Tetracorolla  frequently  shows,  by  means  of  low 
lines  or  low  ridges,  the  number  and  direction  of  the  septa, 
and  in  some  of  the  later  species  these  external  septal  lines 
are  ornamented  with  rows  of  short  spines  or  spinules;  as  in 
CyatJiaxonia  cynodon18  and  Zaphrentis  spi?iulosa.18 

Many  Crinoidea  and  Asteroidea  show  the  development  of 
tubercles  into  spines,  and  the  surface  sculpture  is  often 
made  up  of  ridges  which  bear  strong  spines  at  the  points  of 
intersection;  as  in  Grilbertsocrinus  tulerosus^  Technocrinus 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  51 

spinulosus,^  Actinocrinus   lobatusf9  A.  pernodosus,  Greasier 
oceidentalis^  0.  gigas,  Retaster  cribrosus,  etc. 

The  concentric  laminae  of  growth  in  the  Brachiopoda  are 
frequently  differentiated  into  spinules;  as  in  Siphonotreta 
unguiculata,2*1  Schizambon  typicalis^1  Spirifer  fimbriatus^1 
$.  pseudolineatus^1  $.  setigerus^1  Cliothyris  Royssii^1  etc. 
Other  species  show  the  differentiation  of  the  radii  into 
spines;  as  Acanthothyris  spinosa1*  and  A.  Doderleini.lb  In 
others  the  strong  concentric  laminae  passing  over  radii  are 
often  infolded  into  spines;  as  in  Atrypa  spinosa.31 

Among  the  Mollusca  innumerable  examples  could  be  cited 
showing  clearly  the  differentiation  of  various  ornamental 
features  into  spines.  Some  of  these  40 

have  already  been  discussed,  but 
may  be  referred  to  again  in  this 
connection.  Thus  an  illustration 
of  the  passage  of  concentric  laminae 
into  spines  is  shown  in  Avicula 
sterna^  and  Anomia  aculeata86  (fig- 
ures 26  and  28)  and  Margaritiphora 
fimbriata,  etc.  Many  species  of 
Gastropoda  show  the  same  types  of 
differentiation.  The  differentiation  FlGURB  4o.-Zma  squa- 
of  radiating  lines  or  ridges  into  »*O*M*.  Natural  size.  To  show 


spines  is  equally  common,  and  is  well  e™  aon  of  radii  into 
shown  in  Spondylus  (figures  12,  14, 

30),  and  in  Lima  squamosus  (figure  40).  In  most  of  these 
cases  the  rib  represents  the  progression  of  a  fold  in  the 
edge  of  the  mantle,  while  the  spine  is  a  process  of  a  con- 
centric lamina,  and  is  usually  more  or  less  flat  or  tubular. 
Occasionally  the  rib  becomes  obsolescent,  and  is  represented 
by  a  row  of  spines  ;  as  in  some  specimens  of  the  gastropod 
Crucibulum  spinosum.  When  the  radiating  and  concentric 
ornaments  are  distinctly  continuous,  a  reticulate  or  cancellate 
appearance  is  produced,  and  the  points  of  intersection  often 
bear  spines;  as  in  Aviculopecten  scabridus,™  A.  ornatusf® 
Actinopteria  Boydi^  Pterinopecten  spondylus™  etc. 


52  STUDIES  IN  EVOLUTION 

The  raised  lines  or  ridges  on  the  legs  and  carapaces  of 
Crustacea  are  frequently  spiniferous;  as  Crelasimus  princeps, 
Grecarcinus  ruricola,  etc.  The  radii  on  the  shells  of  barnacles 
are  sometimes  differentiated  into  spines ;  as  in  Balanus  tintin- 
nabulum  var.  spinosus.18 

In  the  higher  animals  the  differentiation  of  ornamental 
features  into  spines  is  not  common,  especially  as  most  of  the 
forms  are  devoid  of  hard  external  parts.  Among  the  fishes 
and  reptiles  certain  lines  and  ridges  on  the  head  and  body 
are  often  spiniferous,  while  in  others  the  scales  have  spi- 
niferous ribs. 

III.    Secondarily  as  a  means  of  protection  and  offence. 
(A,,  B4.) 

After  spines  have  originated  through  the  stimuli  from  the 
environment  acting  on  the  most  exposed  parts,  or  by  growth 
force,  or  by  progressive  differentiation  of  previous  structures, 
they  may  often  acquire  added  qualities,  one  of  which  is  to 
protect  an  organism  from  the  attacks  of  many  of  its  enemies. 

Morris  49  shows  that  defence  in  animals  is  either  mechanical 
or  motor,  while  in  the  higher  plants  it  is  purely  mechanical. 
The  spine  clearly  belongs  to  the  mechanical  mode  of  defence, 
and  in  many  animals  may  be  efficient  without  motion.  If 
motion  is  added,  it  then  may  serve  not  only  for  protection 
but  for  offence  as  well.  Natural  selection  evidently  could 
not  originate  a  spine,  but  after  one  has  appeared  from  any  of 
the  causes  mentioned  in  the  preceding  paragraph,  this  agency 
could  tend  to  preserve  and  allow  the  spine  to  develop  along 
certain  lines.  The  restrictions  as  a  defensive  structure  would 
be  those  of  efficiency,  and  therefore  all  the  monstrous  growths, 
vagaries,  and  ornamental  spine  features  would  arise  indepen- 
dently of  the  action  of  protective  selection,  and  would  be 
accounted  for  by  the  operation  of  the  forces  of  the  environ- 
ment, growth,  and  sexual  selection.  In  this  way  the  simple 
antlers  of  the  Tertiary  Deer  may  be  imagined  to  have  reached 
the  highest  degree  of  efficiency  as  weapons,  by  ordinary  nat- 
ural selection  (figure  41).  In  most  cases  the  subsequent 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


53 


increasing  complexity  of  the  antlers  during  more  modern 
times  cannot  have  improved  their  usefulness  for  protection 
or  fighting  (figures  42,  43),  and  probably  arose  through 
gradual  specialization  according  to  the  law  of  multiplication 
of  effects,  acted  on  by  the  agency  of  sexual  selection.  In 
some  species,  as  the  Reindeer  (Rangifer  tarandus),  the  differ- 
entiation of  the  antlers  has  secondarily  produced  a  useful 
structure.  One  of  the  brow  tines  in  this  species  has  become 
greatly  enlarged  and  palmated,  and  serves  to  assist  in  remov- 
ing the  snow  to  uncover  food.  Evidently  this  has  had  some- 
thing to  do  with  the  common  retention  of  the  antlers  in  both 
sexes. 


41 


42 


43 


FIGURE  41.  — Antler  of  Cervulus  (?)  dicranoceros.     Pliocene. 
FIGURE  42.  —  Antler  of  Cervus  pardinensis.     Pliocene. 
FIGURE  43.  —  Antler  of  the  Fallow  Deer  (Cervus  dama).    Reduced. 
Nicholson  and  Lydekker.) 


(After 


Certain  types  of  horns  are  common  to  particular  regions, 
especially  when  the  cattle  are  in  a  semi-wild  state,  as  in  the 
Western  Plains  of  America.  The  Texas  cattle  have  long, 
gently  curved  horns  standing  out  from  the  head.  Similar 
forms  are  prevalent  in  the  cattle  of  southern  Italy  and  in 
other  warm  temperate  regions.  Further  north,  the  horns 
become  more  curved  in  a  direction  parallel  with  the  head, 
and  are  therefore  closer  to  the  skull.  The  most  northerly 
representative  of  the  hollow-horned  ruminants,  the  Musk-Ox 
(Ovibos  moschatus),  has  the  horns  hanging  down  close  to  the 
skull  and  only  curved  outward  in  their  distal  portions. 


54  STUDIES  IN  EVOLUTION 

Marsh  suggests  to  the  writer  that  these  variations  in  the 
directions  of  the  horns  have  been  influenced  by  the  climate. 
A  warm  climate  permits  the  horns  to  stand  out  directly  from 
the  skull.  Further  north,  or  in  a  colder  region,  the  frequent 
freezing  of  the  horns  and  their  consequent  drooping  has 
induced  a  natural  drooping  condition,  and  an  Arctic  climate 
has  resulted  in  the  production  of  horns  closely  appressed  to 
the  skull,  in  which  position  they  cannot  be  affected  by  freez- 
ing temperatures. 

Another  possible  service  for  antlers  is  also  suggested  by 
Marsh.  As  is  well  known,  the  male  Moose  is  one  of  the 
most  wary  of  the  Cervidse,  and  detects  noises  at  great  dis- 
tances. The  large  palmate  antlers  act  as  sounding-boards, 
and,  when  listening,  the  animal  holds  his  ears  in  the  focus  of 
the  anterior  surfaces  of  the  antlers. 

The  hollow-horned  mammals  afford  some  of  the  most  evi- 
dent examples  of  the  use  of  horns  for  protection  and  offence. 
In  species  with  permanent  horns,  like  the  bison,  oxen,  goats, 
cattle,  antelopes,  etc.,  the  horns  are  generally  present  in  both 
sexes,  though  in  the  males  they  are  often  much  the  larger. 
In  defence,  many  of  the  horned  ruminants  hold  the  head 
down,  thus  protecting  the  nose  and  bringing  the  top  of  the 
skull  into  prominence.  In  this  position  the  horns  are  most 
effective.  A  similar  posture  is  taken  by  the  horned  batra- 
chians  and  lizards. 

The  Porcupine  and  Echidna  rely  largely  on  the  protection 
afforded  by  their  spines,  and  on  this  account  they  are  sluggish 
in  their  movements,  and  make  little  effort  to  escape  approach- 
ing enemies. 

Many  of  the  great  horned  Dinosauria  of  the  Mesozoic  are 
well  provided  with  an  armature  of  protective  plates  and  spines 
on  various  parts  of  the  body.  In  addition  to  an  armature  on 
the  body  Triceratops^  had  three  large  horns  on  the  head, 
one  median  (nasal)  and  two  lateral  (supra-orbital).  These 
were  powerful  offensive  and  defensive  weapons.  There  were 
also  other  small  nodes  and  spiniform  ossicles  around  the 
posterior  crest  of  the  skull  and  on  the  jugals,  forming  a  part 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  55 

of  the  general  armor.  In  Stegosaurus 46  the  efficient  offen- 
sive and  defensive  weapons  were  the  huge  spines  on  the  tail, 
and  it  is  interesting  to  note,  as  a  parallel  to  this  condition, 
that  the  greatest  nerve  centres  were  in  the  sacrum,  and 
therefore  posterior  also. 

No  group  of  vertebrates  shows  such  a  variety  of  protective 
and  offensive  characters  as  the  fishes.  Many  of  the  older 
types  were  heavily  plated,  while  in  others  the  fin  spines  were 
greatly  developed.  Among  modern  forms  the  protective 
character  of  the  spines  is  well  shown  in  types  like  the  Spiny 
Box-fish  Chilomycterus  geometricus  and  Diodon  maculatus. 
A  combination  of  mechanical  and  optical  protection  is  afforded 
in  the  remarkable  Australian  Pipe-fish  Phyllopteryx  eques2'0 
(figure  49).  This  fish  has  numerous  spines  and  ribbon-like 
branching  filaments,  the  former  giving  it  a  mechanical  defence, 
and  the  latter  assisting  in  its  concealment  among  sea-weeds, 
to  which  it  bears  a  striking  resemblance. 

Spines  for  protection  are  extremely  common  among  insects, 
even  in  larval  forms.  They  have  been  so  frequently  noted  as 
to  require  no  elaboration  here.  Packard  54  has  ably  discussed 
the  origin  of  nodes,  tubercles,  and  spines  among  certain 
caterpillars.  Among  the  forms  which  feed  exclusively  at 
or  near  the  ground,  he  finds  the  body  usually  smooth,  while 
those  feeding  on  trees  or  on  both  trees  and  ground  are 
often  variously  spined  and  tuberculated.  These  ornamental 
features  arise  from  the  modification  of  the  piliferous  warts 
common  to  all  lepidopterous  larvae,  and  he  concludes  that  the 
trees  were  more  favorable  for  temperature,  food,  etc.,  than 
the  ground,  and  that  an  increase  of  nutrition  and  growth 
force  led  to  the  hypertrophy  of  these  warts  into  tubercles  and 
spines.  Having  thus  arisen,  they  immediately  became  useful 
for  protection  from  birds  and  parasitic  insects. 

Among  the  Crustacea  there  are  also  numerous  examples 
of  protective  spines.  These  may  be  confined  to  parts  of  the 
body  and  legs  especially  exposed,  or  the  entire  animal  may 
partake  of  the  spiny  character,  as  in  the  crab  Echidnoce- 
ras  setimanus,  where  even  the  eye-stalks  and  antennae  are 


56  STUDIES  IN  EVOLUTION 

spiniferous.  Others,  like  Lithodes  maia,  have  the  spines 
generally  distributed  over  the  carapace  and  legs.  While 
serving  for  defensive  purposes,  this  generally  spinose  char- 
acter has  probably  reached  its  extreme  development  through 
the  influence  of  repetition  (B2).  The  nauplius  larva  of 
Lepas  fascicularis  is  very  large,  and  has  highly  defensive 
spines  which  are  explained  by  Balfour 8  as  a  secondary  ad- 
aptation for  protection.  The  larger  spines  on  Trilobita, 
especially  those  from  the  genal  angles  and  the  axis,  doubtless 
served  protective  purposes.  The  extremes  of  spinosity  in 


FIGURE  44. —  Zoea  of  the  common  crab  (Cancer  irroratus) ;  lateral  view. 
X8.  (After  Verrill  and  Smith68.) 

this  class  are  found  in  the  various  species  and  genera  of 
the  family  Acidaspidae,  and  also  in  many  forms  of  Arges, 
Terataspis,  Hoplolichas,  etc. 

Even  among  the  star-fishes,  which  are  so  generally  spinose, 
some  forms  have  the  spines  so  prominently  developed  on  the 
most  exposed  portions  of  the  animal  that  they  evidently 
serve  for  protection ;  as  Acanthaster  Solaris,  JSchinaster 
spinosus,  etc. 

The  examples  already  given  are  sufficient  to  emphasize  the 
fact  that  after  spines  are  developed  they  may  then  often 
serve  for  protection  and  offence,  and  therefore  be  useful,  their 
efficiency  being  controlled  by  natural  selection  resulting  in 
the  survival  of  the  fittest. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  57 

Another  process  or  kind  of  selection  has  been  described  by 
Verrill  as  "Cannibalistic  Selection."  He  has  shown  that 
the  young  of  carnivorous  animals  often  prey  upon  each 
other,  as  in  the  larval  forms  of  some  Decapoda,  or  sometimes 
even  before  the  escape  of  the  young  from  the  egg  capsules, 
as  in  some  of  the  Gastropoda.  Here,  of  course,  any  natural 
variation  in  the  newly  hatched  animals  which  would  give  an 
individual  some  advantage  over  its  companions  would  tend 
to  its  preservation  and  to  their  destruction.  In  this  way  it 
may  occur  that  the  relative  growth  of  spines  in  the  zoe'a  of 
decapods  has  determined  the  survival  of  the  well-armed 
individuals;  as  in  the  zoe'a  of  Cancer Q*  (figure  44),  Carcinus, 
Homarus,  etc. 


IV.  Secondarily  from  sexual  selection.     (A4,  B4.) 

The  males  and  females  of  so  many  animals  present  dif- 
ferences in  size,  color,  and  ornament,  that  corresponding 
variations  in  the  development  of  spines,  horns,  and  antlers, 
might  naturally  be  expected.  That  such  differences  actually 
occur  in  nature  is  evident.  Every  gradation  can  be  found 
between  horns  or  antlers  common  to  both  sexes  and  those 
confined  to  one  sex.  Probably  the  initial  difference  is  as 
ancient  as  sex  itself. 

Sexual  variations  of  horns  are  most  familiar  among  the 
mammals.  Some,  as  the  Giraffe,  Ox,  Bison,  and  Reindeer, 
have  them  present  in  both  sexes,  though  the  antlers  of  the 
female  Reindeer  are  smaller  and  more  slender  than  in  the 
male,  and  in  the  American  variety  are  sometimes  absent. 
Others,  as  in  the  Prong-horn  Antelope,  many  sheep,  goats, 
etc.,  have  the  horns  usually  quite  small  in  the  female,  and 
well  developed  in  the  male.  Lastly,  the  modern  Deer,  Elk, 
Moose,  etc.,  have  the  antlers  confined  to  the  males  alone,  the 
female  being  entirely  without  them. 

Some  of  the  early  deer  (Procervulus)  seem  to  have  had 
antlers  in  both  sexes,  and  in  nearly  all  the  families  of  the 


58  STUDIES  IN  EVOLUTION 

Ruminata  there  are  species  without  horns,  other  species  with 
horns  in  both  sexes,  and  still  others  with  horns  only  in  the 
male.  In  the  wild  state  the  presence  or  absence  of  horns 
and  their  character  in  any  particular  species  seem  to  be  well 
established,  but  in  domesticated  forms  the  greatest  variety 
is  found.  Among  domesticated  cattle,  presumably  of  one 
species  originally,  varieties  are  found  without  horns,  and 
others ,  with  horns  showing  all  degrees  of  twisting  and 
length. 

tj&r  By  protecting  cattle  from  enemies,  by  forcing  them  into 
}^  A,  changed  environment,  and  by  varying  amounts  of  nutrition, 
$+'  man  has  evidently  brought  the  original  stock  into  a  condition 
j/^  of  free  variation.  This  state  has  been  made  use  of  in  the 
production  of  endless  varieties  by  selection  and  cross-breeding. 
Darwin14  accounts  for  the  sexual  selection  affecting  the 
growth  of  the  antlers  in  the  Deer  as  due  to  excess  in  the 
number  of  male  individuals,  and  their  struggles  for  supremacy 
in  the  possession  of  a  mate.  The  antlers  at  the  breeding 
season  are  strong  and  solid,  and  are  therefore  at  their 
maximum  of  efficiency  in  each  individual.  They  are  shed 
at  or  before  the  time  the  young  are  born.  Previous  to  the 
growth  and  maturity  of  the  new  antlers,  the  young  are  so 
far  advanced  as  to  be  able  to  avoid  being  killed  by  the  adult 
males.  Furthermore,  Darwin  suggests  that  the  excessive 
*  i  development  of  antlers  into  palmate  and  arborescent  forms 
was  probably  an  ornamental  character  attractive  to  the 
females.  These  complicated  antlers  not  being  the  most 
efficient  weapons,  the  fighting  proclivities  of  the  males  would 
tend  to  favor  the  individuals  with  simple  antlers,  and  to 
repress  the  more  differentiated  forms.  Thus  the  two  in- 
fluences would  be  opposed  to  each  other,  though  not  necessarily 
equal.  The  law  of  the  multiplication  of  effects  may  also 
have  some  force,  since  it  may  carry  a  structure  beyond  the 
bounds  of  efficiency.  Even  in  one  of  the  oldest  horned 
mammals,  the  Protoceras 45  of  the  Miocene  Tertiary,  a  great 
difference  is  seen  in  the  horns  of  the  two  sexes.  The  female 
has  little  nodes  or  tubercles,  which  in  the  male  rise  to  the 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  59 

height  and  prominence  of  the  horns  on  the  Giraffe,  or  are 
even  relatively  more  pronounced. 

The  males  of  some  other  vertebrates  have  spiniform  processes 
or  spurs  on  their  legs  and  wings  serving  particular  functions. 
The  spurs  in  birds  are  to  be  considered  mainly  as  weapons 
which  are  used  by  the  males  in  combats  among  themselves. 
They  are  developed  on  the  metatarsal  or  metacarpal  bones  as 
bony  processes  ensheathed  in  horn.  In  the  females  the 
spurs  are  generally  rudimentary.  A  kind  of  spur  is  also 
found  on  the  hind  limbs  of  the  male  Echidna  and  Ornitho- 
rfiynchus,  attached  to  the  astragalus.  It  is  perforated  by  a 
duct  leading  from  a  gland.  The  functions  of  the  spur  and  of 
the  secretion  are  unknown. 

Many  lizards,  especially  among  the  Chamseleontidse,  present 
striking  differences  between  the  sexes,  and  the  males  of  some 
of  them  develop  veritable  horns  like  those  in  cattle,  sheep, 
and  other  hollow-horned  ruminants.  Darwin14  illustrates 
and  describes  a  number  of  most  interesting  examples.  One 
of  them  Chamceleon  Oweni  is  tyere  shown  (figures  45,  46). 
The  male  has  three  horns,  one  on  the  snout  and  two  on  the  - 
forehead.  They  are  supported  by  bony  excrescences  from 
the  skull.  From  the  peaceable  nature  of  these  animals, 
Darwin  concludes  that  "we  are  driven  to  infer  that  these 
almost  monstrous  deviations  of  structure  serve  as  masculine 
ornaments." 

The  males  of  the  tropical  American  genus  of  fishes  Oal- 
lichthys  "have  the  spines  on  the  pectoral  fins  stronger  and 
longer  than  those  of  the  female,  the  spine  increasing  in  size 
as  the  male  reaches  maturity"  (Seeley65). 

Among  insects  the  males  of  many  beetles  belonging  to  the 
lamellicorns  have  long  horns  arising  from  various  parts  of 
the  head  and  thorax.  One  of  the  best  known  forms  is  the 
Hercules  beetle  Dynastes  hercules.  Bateson5  states  that,  in 
this  and  other  genera,  it  is  commonly  found  that  the  males 
are  not  all  alike;  but  some  are  of  about  the  size  of  the 
females  and  have  little  or  no  development  of  horns,  while 
others  are  more  than  twice  the  size  of  the  females  and  have 


60 


STUDIES  IN  EVOLUTION 


45 


enormous  horns.  These  two  forms  of  male  are  called  "  low  " 
and  "high"  males,  respectively.  Among  the  males  sim- 
ilar dimorphism  in  respect  to  size  and  length  of  horns 
occurs  in  Xylotrupes  gideon,  and  in  the  stag  beetle  (Luca- 
nus  cervus,  L.  titanus,  L.  dama). 

In  many  of  these  cases  the  horns 
are  evidently  protective,  and  not  de- 
veloped through  the  selective  influ- 
ences of  the  female.  In  such  cases 
the  habits  of  the  male  are  supposedly 
different  from  those  of  the  female. 
Thus  Wallace70  suggests  that  the 
horned  males  of  the  coleopterid  fam- 
ilies Copridse  and  Dynastidse  fly 
about  more,  as  is  commonly  the  case 
with  male  insects,  and  that  the 
horns  are  an  efficient  protection 
against  insectivorous  birds.  These 
interpretations  clearly  do  not  come 
under  the  definition  of  sexual  selec- 
tion as  restricted  to  the  choice  of 
either  sex.  Beauty,  voice,  or  strength  may  influence  the 
selection  of  a  mate  by  the  opposite  sex,  but  when  the  habits 
of  the  sexes  are  different,  and  certain  characters  arise  in 
response  to  this  change,  the  explanation  is  then  really  found 
in  the  law  of  adaptation  or  physical  selection. 


FIGURE  45.  —  Profile  of 
head  of  Chamceleon  Oweni ; 
male.  \. 

FIGURE  46.  —  Female  of 
the  same  species.  \.  (After 
Darwin.) 


V.    Secondarily  from  mimetic  influences.     (A5,  B4.) 

Natural  selection  may  aid  in  furthering  and  preserving  a 
spinose  organism  after  the  spines  have  originated  through 
any  primary  cause.  One  aspect  of  this  influence  may  be 
treated  under  the  head  of  mimicry.  If,  by  their  resemblance 
in  form,  color,  or  voice,  any  characters  are  similar  to  char- 
acters present  in  the  surroundings  of  the  animal,  and  afford 
a  means  of  protection  or  are  useful,  they  may  be  considered 
as  mimetic  in  the  broadest  sense  of  the  term.  Mimicry  is 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  61 

usually  restricted  to  a  kind  of  special  resemblance,  and  not  to 
the  cases  of  general  resemblance  afforded  by  an  animal  without 
significant  colors  in  general  harmony  with  its  surroundings. 

The  influence  of  mimicry  in  the  production  of  spines  can 
only  occur  where  the  object  mimicked  is  spiniform  or  spinose. 
Apparently  this  is  rather  infrequent  and  of  little  real  impor- 
tance as  a  factor  of  acanthogeny. 

Insects  and  spiders  have  furnished  the  greatest  number 
and  variety  of  mimetic  forms,  both  in  their  larval  and  adult 
conditions,  and  naturally  would  be  expected  to  furnish  ex- 
amples of  spines  having  mimetic  significance.  The  object 
mimicked  may  be  another  species  of  insect  or  animal,  in 
which  case  there  is  usually  some  offensive  or  defensive 
quality  rendering  the  resemblance  useful  to  the  mimicker; 
or  the  whole  or  a  portion  of  some  plant  or  other  object  may 
be  imitated,  tending  to  the  more  or  less  complete  conceal- 
ment of  the  mimicking  insect. 

Satisfactory  examples  are  not  at  hand,  though  doubtless 
many  occur  in  nature,  and  some  Jiave  been  described,  but  not 
for  the  present  purpose.  A  few  will  be  cited  here  which 
seem  to  conform  to  the  requirements. 

47 


FIGURE  47. —  Profile  of  a  spider  (Ccerostris  mitralis)  on  a  twig  mimicking  a 
spiny  excrescence.  (From  Peckham,  after  Vinson.) 

FIGURE  48.  —  The  larva  of  the  Early  Thorn  Moth  (Selenia  iHunaria)  resting 
on  a  twig;  showing  mimicry  of  stem  and  spiniform  processes.  ^.  (After 
Poulton.) 

A  Madagascar  spider  (^Ccerostris  mitralis)  is  described  by 
Elizabeth  G.  Peckham66  as  sitting  motionless  on  a  branch 


62 


STUDIES  IN  EVOLUTION 


and  resembling  a  woody  excrescence  with  projections  or 
spiniform  processes  (figure  47).  Other  spiny  spiders  of  the 
Epeiridse  probably  have  similar  protective  mimetic  features  ; 
as  Epeira  spinea  and  Acrosoma  arcuata. 

The  larva  of  the  Early  Thorn  Moth,  as  described  and  illus- 
trated by  Poulton,58  bears  a  strong  resemblance  to  the  twig 
upon  which  it  rests,  even  to  the  spiniform  processes,  axils,  and 
buds  (figure  48).  Packard 54  cites  a  striking  case  of  mimicry 


49 


FIGURE  49.  —  Australian  Pipe-fish  (Phyllopteryx  eques)  and  frond  of  sea-weed 
in  lower  right-hand  corner ;  showing  mimicry.    £.     (After  Gunther.) 

in  the  caterpillar  of  another  genus  of  moth  (ScUzurd),  where 
the  spines  and  tubercles  resemble  the  serrations  of  a  leaf 
"so  that,  when  feeding  on  the  edge  of  a  leaf,  the  Schizurse 
exactly  imitate  a  portion  of  the  fresh-green  serrated  edge  of 
a  leaf  including  a  sere,  brown,  withered  spot,  the  angular, 
serrate  outline  of  the  back  corresponding  to  the  serrate  out- 
line of  the  edge  of  the  leaf." 

The  Australian  Pipe-fish    Phyllopteryx,    previously   men- 
tioned under  the  head  of  spines  for  protection,  shows  the 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  63 

mimicry  of  a  plant  by  an  animal  to  a  striking  degree.  This 
fish  closely  imitates  a  sea-weed  (figure  49),  and  Giinther25 
gives  the  following  description  of  the  spines  and  filaments 
on  the  species  Phyllopteryx  eques :  "  There  is  a  pair  of  small 
spines  behind  the  middle  of  the  upper  edge  of  the  snout,  a 
pair  of  minute  barbels  at  the  chin,  and  a  pair  of  long  appen- 
dages in  the  middle  of  the  lower  part  of  the  head.  The 
forehead  bears  a  broad,  erect,  somewhat  four-sided  crest, 
behind  which  there  is  a  single  shorter  spine.  A  horizontal 
spine  extends  above  each  orbit.  There  is  a  cluster  of  spines 
on  the  occiput,  and  from  these  narrow  appendages  are  pro- 
longed. On  the  nape  of  the  neck  is  a  long  spine,  dilated  at 
the  base  into  a  crest,  and  carrying  a  long  forked  appendage. 
The  back  is  arched,  and  on  the  under  side  are  two  deep 
indentations.  The  spines  on  the  ridges  of  the  shields  are 
the  strongest;  they  are  compressed,  are  not  flexible,  and 
each  terminates  in  a  pair  of  short  points.  There  is  one  pair 
of  these  spines  in  the  middle  of  the  back,  and  one  on  each  of 
the  three  prominences  of  the  abcjominal  outline ;  they  termi- 
nate in  flaps,  which  are  long  and  forked.  There  are  also 
very  long  compressed  flexible  spines  without  appendages, 
which  extend  in  pairs  along  the  uppermost  part  of  the  back, 
while  a  single  series  extends  along  the  middle  line  of  the 
belly.  Small  short  conical  spines  run  in  a  single  series  along 
the  middle  line  of  the  sides,  and  along  the  lateral  edges  of 
the  belly ;  and  there  is  a  pair  of  similar  spines  in  front  of  the 
base  of  the  pectoral  fin.  The  tail,  which  is  about  as  long  as 
the  body,  carries  the  dorsal  fin ;  it  is  quadrangular,  and  has 
sharp  edges.  It  carries  along  its  upper  side  five  pairs  of  band- 
bearing  spines,  which  terminate  in  branching  filaments." 

The  Horned  Toad  Phrynosoma  bears  considerable  resem- 
blance to  the  joints  of  the  Prickly  Pear,  with  which  it  is 
often  associated,  and  it  may  be  suggested  that  the  likeness 
both  in  form  and  spinescence  represents  mimetic  characters. 

*  The  artist  who  copied  Giinther's  figure  for  Leunis'  "  Synopsis  der  Thier- 
kuude,"  3d  ed.,  by  H.  Ludwig  (vol.  i.  p.  770, 1883),  connected  the  fish  with  the 
adjacent  fronds  of  sea-weed  so  as  to  form  a  single  organism. 


64  STUDIES  IN  EVOLUTION 

VI.    Prolonged  development  under  conditions  favorable 
for  multiplication.    (Bi.) 

The  prolonged  development  or  existence  of  a  stock  under 
favorable  conditions  for  multiplication  may  be  considered  as 
one  of  the  primary  influences  favoring  the  production  of 
spines.  This  implies  abundance  of  nutrition  and  compara- 
tively few  enemies  outside  of  other  individuals  of  the  same  or 
closely  related  species.  Under  a  proper  amount  of  increased 
nutrition  the  vitality  and  reproductiveness  of  a  stock  are 
raised,  and,  other  things  being  favorable,  it  is  found  that  the 
stock  will  give  expression  to  what  has  already  been  described 
as  free  variation.  Hypertrophy  is  also  very  apt  to  be  one 
result  of  abundant  nutrition,  so  that  structures  of  little  or 
no  use  may  be  developed,  and  some  of  them  comprise  certain 
features  which  are  often  called  ornamental. 

In  the  excessive  multiplication  of  individuals  it  is  evident 
that  there  must  be  a  great  number  of  natural  variations,  and 
that  some  of  these  will  affect  the  pairing  of  the  sexes  in  such 
a  manner  as  to  accentuate  and  delimit  certain  variations. 
Eventually  there  also  comes  a  struggle  for  existence  in 
which  favorable  modifications  have  a  decided  advantage.  In 
this  way  it  is  believed  that  the  great  amount  of  differen- 
tiation found  in  some  isolated  stocks  has  been  brought 
about.  Primarily,  then,  a  favorable  condition  for  nutrition 
is  assumed,  which  is  followed  by  excessive  numerical  multi- 
plication; while  the  natural  variations  are  augmented  and 
governed  by  the  action  of  reproductive  divergence  for  which 
such  conditions  are  favorable.  Secondarily,  these  variations 
are  subjected  to  the  influences  of  cannibalistic  selection, 
defence,  offence,  sexual  selection,  and  mimicry. 

In  illustration  of  the  amount  of  differentiation  attained  by 
a  single  stock  under  favorable  conditions,  the  Amphipod 
Crustaceans  G-ammarus  and  Allorchestes,  found  in  lakes 
Baikal  and  Titicaca,  respectively,  may  again  be  noticed. 

In  respect  to  the  number  of  species,  G-ammarus  is  very 
sparsely  distributed  over  the  world,  though  in  Lake  Baikal 


ORIGIN  AND  SIGNIFICANCE  OF  SPINES  65 

alone  a  hundred  and  seventeen  species  have  been  described  "^J 
by  Dybowsky.17  In  contrast  to  this,  it  may  be  mentioned 
that  but  four  freshwater  species  have  been  discovered  in  the 
whole  of  Norway.  In  Lake  Baikal  all  the  depths  explored 
(to  1,373  metres)  have  furnished  species.  Those  living  near 
the  surface  are  vividly  colored,  yet  apparently  make  no 
attempts  at  concealment.  Many  of  the  species  are  also 
highly  spinose,  though  not  sufficiently  armed  to  be  protected 
from  the  fish.  As  these  Crustaceans  are  voracious  creatures, 
the  spinose  character  has  probably  been  favored  by  the  agency 
of  cannibalistic  selection.  The  lake  has  a  number  of  species 
of  fish  for  which  the  Gammaridse  furnish  excellent  food,  but 
the  presence  of  a  species  of  seal,  predaceous  fish,  as  well  as 
the  native  fishermen,  keep  the  fish  below  the  danger  point, 
thus  allowing  the  GammaridaB  to  become  very  abundant. 

Similarly,  in  Lake  Titicaca  there  50 

is  a  wonderful  specific  development 
of  a  kindred  Crustacean  Allorches- 
tes.  One  of  the  most  spinose  spe- 
cies (A.  armatus)  is  also  the  com- 
monest, and  according  to  Faxon19 
occurs  in  countless  numbers  (fig- 
ure 50). 

Packard54  shows  that,  among 
certain  moths,  the  caterpillars  as 
soon  as  they  acquired  arboreal  hab- 
its met  with  favorable  conditions 
in  respect  to  food,  temperature, 

etc.,  and  that  as  spines  and  tuber-       FIGURE  50.- Alhrchestes  ar- 
cles  arose  by  normal  variation,  such    matus,  a  spiny  amphipod  from 
features,    being    found    useful   for 
protection,  were  therefore  preserved    (After  Faxon.) 
and  augmented. 

The  differentiation  of  Achatinella  has  already  been  dis- 
cussed (p.  36)  as  affording  a  striking  instance  of  free  varia- 
tion among  the  Mollusca.  The  evolution  of  the  Tertiary 
species  of  Planorlis  at  Steinheim,  as  described  by  Hyatt,35 

5 


66  STUDIES  IN  EVOLUTION 

furnishes  another  example,  though  in  neither  case  has  the 
differentiation  of  structures  proceeded  far  enough  to  result 
in  spines.  The  costate  form  (Planorbis  costatus)  was  tending 
toward  that  end,  but  did  not  attain  it. 

The  series  of  Slavonian  Paludina  in  the  Lower  Pliocene, 
as  elucidated  by  Neumayr  and  Paul,50  shows  a  somewhat 
further  advancement.  The  species  in  the  lowest  beds  (typus 
Paludina  Neumayri)  are  smooth  and  unornamented.  Higher 
in  the  strata  they  are  angular  and  carinated,  and  at  the  top 
of  the  series  the  shells  are  carinated,  nodose,  and  sub-spinose 
(typus  Paludina  Hoernesi).  The  living  American  genus 
Tulotoma  is  closely  related  to  the  most  differentiated  species 
(P.  Hcernesi),  and  its  approach  to  spinose  features  is  more 
pronounced. 

Under  the  phylogeny  of  spinose  forms  (pp.  23-25)  an 
outline  of  the  life  history  of  the  brachiopod  Atrypa  reticularis 
and  derived  species  was  presented.  This  being  one  of  the 
commonest  types  of  Brachiopoda  in  the  Silurian  and  Devo- 
nian, often  forming  beds  of  considerable  extent,  it  seems 
quite  likely  that  its  prolonged  development  under  favorable 
conditions  for  multiplication  must  have  had  an  effect  on  the 
amount  and  kind  of  variation. 

It  has  been  noticed  by  Brady 9  and  others,  that  in  the 
Foraminifera,  Crlobigerina  bulloides,  Orbulina  universa,  etc., 
the  pelagic  forms  comprise  two  varieties  which  are  generally 
distinct,  a  spinous  form  and  another  with  small  minutely 
granular  shells.  The  bottom  specimens  of  the  same  species 
are  also  commonly  without  spines  and  often  smaller.  The 
interpretation  seems  to  be  that  the  large  specimens  indicate 
an  abundance  of  nutrition  which  has  also  produced  hyper- 
trophy of  the  normal  granules  into  spines.  Some  bottom 
specimens  are  large,  but  they  are  usually  abnormal  and  of  a 
monstrous  or  pathologic  nature. 

From  the  foregoing  examples  the  conclusion  to  be  drawn 
is  that,  with  full  nutrition,  there  comes  a  numerical  maxi- 
mum, and  naturally  with  this  a  corresponding  number  of 
normal  variations.  Some  of  these  modifications,  as  spines, 


ORIGIN  AND  SIGNIFICANCE  OF  SPINES  67 

have  arisen  by  hypertrophy.  After  having  thus  originated 
by  growth  force,  they  may  or  may  not  be  of  use  for  offence, 
defence,  or  concealment,  or  in  any  way  give  their  possessor  a 
distinct  advantage. 

VII.    By  repetition.     (B2.) 

Under  the  consideration  of  spine  production  by  repetition 
it  is  proposed  to  include  local  repetition  or  duplication  of 
spines  on  or  about  a  primary  spine,  the  limit  of  this  repetition 
resulting  in  a  generally  spinose  condition. 

It  has  been  shown  that  intermittent  stimulus  produces 
growth,  and  furthermore  that  growth  can  take  place  only 
with  proper  nutrition.  Under  local  stimulus  the  currents  of 
the  circulation  or  forces  of  nutrition  are  set  up  in  an  organism 
toward  the  centre  of  stimulation.  The  nutrient  matter  is 
brought  to  this  point,  and  more  or  less  of  it  is  expended  in 
building  up  a  structure  which  is  the  reciprocal  or  direct 
resultant  of  the  stimulus.  Now,  since  all  motion  is  primarily 
rhythmic,66  and  the  repetition  of  parts  an  almost  universal 
character  among  organisms,5  it  would  appear  that  the  fore- 
going conditions  would  be  favorable  to  the  repetition  or 
reproduction  of  the  structures.  In  this  way  it  is  easy  to 
account  for  the  growth  of  spines  that  cannot  be  explained  as 
the  direct  result  of  external  stimuli  (A),  or  by  any  process 
of  decrescence  (C,  D).  The  nature  of  the  influence  seems  to 
be  similar  to  induction  in  electrical  physics,  or  to  the  force  or 
stimulus  of  example  in  human  conduct. 

Stated  as  a  concrete  case,  a  simple  spine  produced  by  any 
primary  cause  may  be  taken,  and  it  will  be  granted  that  the 
vital  or  physiological  adjustments  produced  in  its  growth 
and  maintenance  have  brought  about  or  induced  an  harmonic 
condition  in  the  adjacent  tissues.  Subsequent  growth  will 
most  naturally  repeat  the  previous  structures,  so  that  in 
addition  to  the  primary  spine  there  will  be  other  smaller 
spines  on  or  about  it,  together  constituting  either  a  com- 
pound spine  or  a  group  of  spines. 


68  STUDIES  IN  EVOLUTION 

Carrying  this  repetitionary  process  to  a  maximum,  there 
would  result  a  generally  spinous  condition.  As  a  possible 
illustration  of  this,  no  class  of  organisms  probably  exhibits  so 
many  kinds  and  series  of  repetitions  of  all  sorts  of  external 
structures  as  the  Echinodermata,  and  it  is  significant  that 
this  is  a  typically  spiniferous  sub-kingdom. 

Except  in  a  few  classes  of  organisms,  compound  spines  are 
relatively  rare  as  compared  with  simple  spines.  They  are 
very  common  among  the  Radiolaria,  which  furnish  the 
greatest  complexity  occurring  anywhere  in  the  organic 
world.  (See  Plate  I.)  They  are  also  quite  frequent  among 
the  Echinoidea,  but  more  rare  among  the  Asteroidea  and 
Crinoidea. 

Compound  antlers  are  especially  characteristic  of  the  mod- 
ern Deer  family,  though  compound  horns  are  but  rarely 
found  elsewhere  among  the  mammals.  The  Prong-horn 
Antelope  of  America  is  the  only  living  species  of  hollow- 
horned  ruminant  having  this  character.  It  of  course  is  not 
intended  that  extra  pairs  of  horns,  which  being  separate,  and 
often  originating  on  different  portions  of  the  skull,  should  be 
considered  as  compound  horns  in  the  sense  here  employed. 
Likewise  compound  spines  arising  through  suppression  of 
organs  or  structures  are  not  to  be  included  here;  as  the 
compound  thorns  on  the  Honey-locust  representing  aborted 
branches. 

The  fin  spines  of  fishes  are  often  compound,  and  sometimes 
are  made  up  of  several  elements ;  as  in  the  spines  of  Edestus 
(E.  vorax).  Quite  a  number  of  Mollusca  develop  compound 
spines ;  as  in  many  species  of  Spondylus  and  Murex.  They 
are  also  not  uncommon  among  the  Crustacea  and  Insecta. 
Compound  spines  are  infrequent  in  the  Brachiopoda,  being 
developed  in  but  few  species  {Spirifer  hirtus 31).  The 
Foraminifera  also  present  but  few  examples  (Polymorphina 
Orbignii  9). 

A  number  of  generally  or  highly  spinose  types  will  now 
be  noted  to  illustrate  the  limits  of  the  repetition  of  spiny 
structures,  the  first  spines  having  probably  arisen  through 


ORIGIN  AND  SIGNIFICANCE  OF  SPINES 


69 


the  operation  of  some  primary  cause,  and  the  derived  or 
secondary  spines  being  produced,  it  is  believed,  by  the  law 
of  repetition. 

The  Radiolaria  have  already  been  frequently  mentioned, 
but  as  they  are  the  most  spiniferous  of  all  classes  of  animals, 
and  represent  the  highest  degree  of  spine  differentiation 
attained  (figure  51  and  Plate  I),  another  brief  notice  will 


51 


52 


FIGURE  51.  —  Acontaspis  hastata,  a  radiolarian ;  showing  multiplication  of 
spines  by  repetition.  X  200.  (After  Haeckel.) 

FIGURE  52.  —  Strophalosia  keokuk,  an  attached  brachiopod ;  showing  the 
spines  extending  from  the  ventral  valve  to  and  along  the  surface  of  attachment. 
X2. 

FIGURE  53.  — A  gastropod  shell  (Platyceras)  to  which  are  attached  a  number 
of  Strophalosia  keokuk.  Natural  size. 

be  of  interest.  These  spines  furnish  characters  of  high 
taxonomic  value,  although  generally  speaking  they  seldom 
have  more  than  specific  importance  among  other  classes. 
The  Echinoidea  and  Asteroidea  must  also  be  noticed  in  this 
connection,  though  from  the  nature  and  origin  of  their  spines 
they  do  not  conform  to  the  mode  of  spine  growth  in  other 
classes. 

Productus,  Productella,  Strophalosia^  Aulosteges,  and  Sipho- 


70  STUDIES  IN  EVOLUTION 

notreta  represent  highly  spinose  genera  among  the  Brachi- 
opoda.  Strophalosia  is  a  form  in  which  the  ventral  valve  is 
cemented  to  some  object.  Whenever  the  valve  rises  well 
above  the  object  of  support,  the  spines  are  free  like  those 
frequently  present  on  the  dorsal  valve ;  otherwise  the  spines 
extend  root-like  along  the  supporting  surface  (figures  52,  53). 

Aulosteges  presents  a  still  further  tendency  to  complete 
spinosity,  for  not  only  are  both  valves  covered  with  spines, 
but  the  deltidium  also. 

Spondylus  (figure  30)  and  Murex  are  well-known  types  of 
very  spiny  forms  of  Mollusca.  Acidaspis,  Terataspis,  etc., 
hold  the  same  place  among  the  Trilobita;  Ecliidnoceras, 
Lithodes,  etc.,  among  the  Decapoda;  and  the  Spiny  Box-fish 
(Diodori),  Pipe-fish,  etc.,  among  the  Pisces.  The  higher 
animals  also  furnish  examples  of  extreme  spinosity;  as  in 
the  Horned  Toad  (Phrynosoma),  the  genera  of  Ceratop- 
sidee,  gigantic  Cretaceous  Dinosauria,  and  the  Echidna  and 
Porcupine. 

All  these  forms  present  numerous  spines,  some  of  which 
cannot  be  explained  as  having  arisen  directly  from  external 
stimuli,  for  they  are  in  comparatively  well-protected  regions 
out  of  the  way  of  external  stimuli.  Neither  can  all  of  them 
serve  for  offence  and  defence,  as  they  are  often  not  located 
in  the  most  advantageous  positions;  nor  are  they  differen- 
tiated out  of  any  previous  ornaments  or  special  structures. 
In  fact,  no  factor  of  spine  genesis  except  the  one  of  repetition 
seems  to  be  sufficient  to  account  for  their  development. 

VIII.    Restraint  of  environment  causing  suppression  of 
structures.     (Ci.) 

The  previous  categories  of  spine  production  (I- VII)  have 
been  brought  about  by  some  process  of  growth  or  concres- 
cence through  external  and  internal  agencies.  There  still 
remains  for  discussion  the  formation  of  spines  by  processes  of 
decrescence  caused  by  extrinsic  restraint  (C)  or  by  intrinsic 
deficiency  of  growth  power  (D).  The  lack  of  vitality  or 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  71 

growth  force  generally  stands  so  directly  as  the  result  of  an 
unfavorable  environment,  that  it  is  often  difficult  or  impos- 
sible to  distinguish  between  their  action.  Furthermore,  as 
in  the  case  of  many  parasites,  it  may  be  seen  that  the  envi 
ronment  may  be  quite  favorable  as  regards  temperature, 
nutrition,  etc. ;  but  unfavorable  in  respect  to  motion  and  use 
of  sensory  and  motive  organs.  From  the  almost  universal 
degradation  and  retrogression  of  parasitic  forms,  it  is  neces- 
sary to  consider  these  as  intrinsically  deficient,  and  therefo 
lacking  in  the  qualities  ofgrowtETIbrce  which  normally  favor^. 
a  progressive  evolution.  Here,  also,  there  are  apparently 
two  intimately  associated  causes.  In  an  attached  animal 
the  absence  of  stimulus  from  disuse  of  an  organ  tends  toward 
atrophy,  and  the  retrogressive  development  serves  to  affect 
many  organs  in  the  same  manner.  The  direct  and  indirect 
results  of  the  restraint  of  the  environment  may  therefore  be 
expected  to  shade  imperceptibly  into  each  other,  with  only 
the  extremes  sufficiently  distinct  for  separation. 

The  influence  of  an  unfavorable  environment  as  affecting 
the  character  and  growth  of  plants  and  animals  is  well  shown 
in  desert  or  arid  regions,  and  the  flora  has  been  made  the 
subject  of  especial  study  by  Henslow.83  In  such  regions 
the  first  thing  to  impress  the  observer  is  the  small  size  of  the 
species.  Next  to  diminutive  size,  the  scantiness  of  life  is  a 
striking  feature,  for  large  areas  are  common  in  which  life  is 
almost  wanting.  An  examination  of  these  plants  reveals  a 
series  of  characters  not  usually  present  elsewhere,  among 
which  may  be  mentioned  the  development  of  a  minimum 
amount  of  surface,  constituting  what  is  known  as  consoli- 
dated vegetation;  next  their  uniform  gray  color,  due  either 
to  excessive  hairiness  or  a  coating  of  wax ;  and  lastly,  their 
frequent  spinescent  characters. 

The  spines  on  desert  plants  are  a  feature  of  such  general 
occurrence  that  it  has  led  to  the  notion  that  vegetable  spines 
are  always  associated  with  unfavorable  conditions  and  are 
therefore  suppressed  structures.  This  is  probably  incorrect* 


72  STUDIES  IN  EVOLUTION 

for  in  plants,  as  in  animals,  spines  may  be  developed  by  the 
progressive  differentiation  of  previous  structures;  as  in  the 
angular  edges  of  the  leaf  stems  of  many  Palms  becoming 
spiniferous,  or,  as  will  be  shown,  suppressed  structures  may 
arise  from  deficiency  of  growth  force.  In  all  cases  spines 
may  or  may  not  serve  for  protection.  Thus,  while  they  are 
not  always  an  indication  of  unfavorable  environment,  those 
occurring  on  desert  plants  may  generally  be  so  considered, 
for  they  are  developed  out  of  structures  which  are  normally 
of  vital  physiological  importance. 

An  animal  or  plant  having  spines  and  living  in  a  favorable 
environment,  involving  freedom  of  motion  for  animals  and 
abundance  of  nutrition  without  extremes  of  temperature  or 
dryness  for  both  animals  and  plants,  will,  it  is  believed, 
from  the  discussions  and  analyses  of  spine  genesis  in  its 
various  phases,  develop  these  features  in  most  instances  with- 
out the  sacrifice  of  organs  and  structures  having  important 
physiological  and  motor  functions.  Thus,  ordinarily,  among 
animals  it  is  found  that  spines  arise  as  excrescences  or  out- 
growths of  exoskeletal  or  epidermal  tissues,  without  seriously 
affecting  the  function  of  the  organ  or  organs  upon  which 
they  are  located.  Such  cases  may  clearly  belong  to  the  most 
progressive  series,  and  in  fact  usually  occur  there. 

On  the  other  hand,  if  it  is  found  that  a  leg,  a  wing,  a 
digit,  or  other  organ  is  developed  into  a  spine,  this  is  always 
accomplished  by  a  process  of  retrogression,  resulting  in  the 
greater  or  lesser  suppression  of  the  part  in  question.  It  is 
also  seen  that  this  kind  of  spine  occurs  most  frequently  in 
retrogressive  series  or  in  others  showing  arrested  develop- 
ment, and  the  necessary  interpretation  seems  to  be  either  that 
the  environment  is  or  has  been  unfavorable,  at  least  so  far  as 
the  particular  organ  or  set  of  organs  is  concerned,  or  that  the 
vital  power  has  declined.  Both  influences  are  intimately 
associated,  and  the  latter  is  often  the  direct  result  of  the 
former. 

The  stunting  effects  of  aridity  and  barren  soil  on  com- 
mon plants  is  familiar  to  all.  Among  the  plants  of  the 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  73 

desert  is  found  every  evidence  of  similar  stunting  combined 
with  adaptations  to  resist  the  unfavorable  conditions  of  defi- 
cient water  supply,  excess  of  radiation,  etc.  The  diminution 
in  size  applies  not  only  to  stature,  but  to  the  leaves  and 
branches,  especially  the  parenchymatous  tissues  or  parts  of 
the  plant  engaged  in  aerial  assimilation.  Consonant  with 
these  changes,  the  drought  and  other  conditions  produce  a 
hardening  of  the  mechanical  tissues,  which  is  of  great  aid  in 
resisting  the  extreme  heat  and  dryness  of  the  desert.  Some- 
times a  deposit  of  wax  affords  a  similar  protection. 

The  reduction  of  the  leaves  takes  place  in  various  ways. 
They  may  simply  become  smaller  in  every  dimension  and 
finally  be  reduced  to  mere  scales,  or  an  aphyllous  condition 
may  be  established.  They  may  grow  narrower  and  narrower 
until  only  the  hardened  veins  or  midrib  remain;  or  leaves 
may  be  developed  only  for  a  short  time,  and  in  the  case  of 
compound  leaves  after  the  shedding  of  the  leaflets  a  spini- 
f  orm  leaf  axis  remains ;  as  in  Astragalus  Tragacantha  ®  (figures 
55,  56).  The  suppression  of .,  branches  tends  toward  the 
same  end ;  namely,  either  to  their  complete  disappearance  or 
to  their  partial  suppression  into  hard  spiniform  processes  or 
thorns.  Thus  leaves,  branches,  and  other  parts  of  the  plants 
may  become  reduced  to  their  axial  elements,  bringing  about 
what  is  commonly  termed  spinescence. 

The  spiny  character  of  these  plants  is  therefore  one  of  the 
results  of  an  arid  environment,  and  it  may  or  may  not  be  of 
sufficient  frequency  to  give  an  especial  character  to  a  partic- 
ular desert  flora.  There  is,  moreover,  a  secondary  influence 
which  has  an  effect  in  determining  the  abundance  of  spinose 
plants  in  desert  as  well  as  in  many  other  situations.  This 
relates  to  the  destruction  of  the  edible  unarmed  species  by 
herbivorous  animals,  and  the  comparative  immunity  of  the 
spiny  types.  Thus,  in  old  pastures,  the  prevailing  flora  is 
apt  to  be  one  that  is  offensive  to  grazing  animals.  This 
character  is  generally  given  by  poisonous  plants  or  those 
having  a  disagreeable  flavor,  or  by  those  whose  form  or  spiny 
structures  afford  protection. 


74  STUDIES   IN  EVOLUTION 

This  secondary  influence  by  grazing  animals  may  have  had 
some  effect  in  determining  the  particular  abundance  of  spiny 
plants  in  certain  desert  regions,  and  their  comparative  infre- 
quency  in  other  similar  regions.  In  either  case  the  unfavor- 
able environment  brings  about  a  suppression  of  structures, 
and  one  type  of  this  action  results  in  the  production  of 
spines.  These  represent  the  limits  of  retrogression  before 
the  part  becomes  entirely  obsolete. 

Wallace  has  criticised  Henslow's  views  on  the  origin  of 
xerophilous  plants  and  their  distribution.  It  is  believed 
that  the  views  here  offered  remove  some  of  the  objections, 
and  bring  the  opinions  of  these  authors  into  greater  accord. 

Under  arid  conditions  bracts,  stipules,  leaves,  and  even 
branches  may  become  spinescent.  Some  forms  in  which  the 
spinose  character  has  not  as  yet  become  permanently  fixed  by 
heredity,  when  transported  or  found  living  in  moister  and 
richer  soils,  develop  normal  leaves  or  branches,  and  lose  their 
spinescence ;  others,  like  the  Cactus,  retain  their  spines  under 
similar  changes;  while  still  others,  as  Acanthosicyos  hor- 
rida,^  cannot  be  artificially  cultivated,  and  have  become 
truly  xerophilous  types. 

As  examples  of  plants  which  lose  their  spines  by  cultiva- 
tion, the  Pear,  species  of  Rose,  Plum,  etc.  (Henslow),  may 
be  cited.  According  to  Henslow,83  others,  as  Onomis  spinosa, 
have  an  especially  spiny  variety  (horrida)  living  on  sandy 
sea-shores,  while  in  more  favorable  natural  situations  the 
same  plant  becomes  much  less  spiny,  and  under  cultivation 
loses  its  spines.  M.  Lothelier 42  also  found  that  by  growing 
the  Barberry  (Berberis  vulgaris)  in  moist  air,  the  spines  dis- 
appeared, the  parenchyma  of  the  leaves  being  well  formed 
between  the  ribs  and  veins.  Dry  atmosphere  and  intense 
light  both  favored  the  production  of  spines. 

Henslow 33  cites  the  genus  Zilla  as  a  desert  plant  in  which 
the  branches  are  transformed  into  spines,  EMnops  for  a 
similar  modification  of  the  foliage,  Fagonia  for  spiniform 
stipules,  and  Centaur ea  for  spinescent  bracts.  As  further 
illustrations  taken  not  only  from  desert  plants  but  also  from 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


75 


others  commonly  found  in  dry,  rocky,  or  unfertile  situations, 
the  following  examples  may  be  taken,  some  of  which  are 
familiar  cultivated  species:  The  stunting  of  branches  into 
spines  is  common  among  neglected  Pear  and  Plum  trees, 
and  is  a  normal  character  in  the  Hawthorn,  Honey-locust, 
Oytisus  (figure  54),  Vella,  etc.  Leaves  transformed  into 
spines  are  characteristic  of  the  Cactacese  of  America,  the 
columnar  Euphorbiacese  of  Africa  and  southern  Asia,  and 
are  also  familiar  in  the  half-shrubby  Tragacanth  bushes 
(figures  55,  56)  so  common  in  southern  Europe,  especially  in 
the  eastern  portion,  and  in  the  ordinary  Barberry  (figure  13). 
Spiniform  stipules  are  usually  present  in  the  species  of 
Rolinia,  of  which  the  Common  Locust  (Robinia  Pseudacacia) 
furnishes  a  well-known  illustration  (figure  57).  Spiniform 
bracts  are  best  known  among  the  Thistles  (CHrsium  lanceo- 
latum,  C.  horridulum,  etc.). 


54 


55 


56 


57 


FIGURE  54.  —  The  spiny  Cytisus  (C.  spinosus) ;  showing  suppression  of  branches 
into  spines.  (After  Kerner.) 

FIGURE  55.  —  A  single  leaf  of  Tragacanth  (Astragalus  Tragacantha),  from 
which  the  three  upper  leaflets  have  fallen.  (After  Kerner.) 

FIGURE  56.  —  Leaf  axis  of  the  same,  from  which  all  the  leaflets  have  fallen. 
(After  Kerner.) 

•    FIGURE  57.  — Twig  of  Common  Locust  (Robinia  Pseudacacia) ;  showing  spines 
representing  stipules. 

As  the  restraint  of  an  environment  acting  on  an  animal 
so  generally  results  in  the  disuse  and  atrophy  of  the  organs 
affected,  most  cases  will  have  to  be  considered  under  the 
head  of  disuse.  Therefore,  while  the  environment  is  the 


76  STUDIES  IN  EVOLUTION 

primary  factor,  its  influences  are  mainly  exhibited  through 
secondary  or  resultant  conditions.  In  some  cases,  however, 
it  is  possible  to  interpret  a  vestigial  or  suppressed  structure 
directly  into  terms  of  an  unfavorable  environment.  Thus, 
if  the  probable  origin  of  the  vestigial  hind  legs  of  a  Python 
is  considered,  it  leads  to  the  belief  that  they  represent  legs 
which  were  of  functional  importance  to  some  of  the  early 
ancestors  of  this  snake.  The  gradual  elongation  of  the  body 
and  the  consequent  change  from  a  walking  or  direct  crawling 
habit  to  a  mode  of  progression  chiefly  by  horizontal  undula- 
tions, necessarily  brought  the  legs  into  a  relation  with  the 
environment  which  was  unfavorable  either  for  their  function 
or  growth.  Their  suppression  is  complete  in  most  snakes, 
but  in  the  Python  the  hind  legs  are  represented  by  two  spurs 
or  spines  (figures  58  and  59).  On  the  interior  of  the  body 
they  are  supported  by  vestiges  of  femora  and  ilia,  showing 
their  true  affinities  with  hind  limbs.  Some  snake-like  batra- 
chians,  as  Amphiuma  and  Proteus,  still  retain  short  and  weak 
external  limbs.  These  would  undoubtedly  soon  be  lost  by  a 
change  from  aquatic  to  terrestrial  or  arboreal  habits. 

58  59 


FIGURE  58.  —  Portion  of  skin  of  Python ;  showing  the  spurs  which  represent 
the  suppressed  or  vestigial  hind  legs.  X  \-  (After  Romanes.) 

FIGURE  59.  —  Bones  of  suppressed  legs  of  Python.  All  but  the  claw-like 
termination  are  internal.  X  \.  (After  Romanes.) 

In  explanation  of  the  nodes  and  spiniform  processes  on  the 
epitheca  of  Michelinia  favosa,  it  may  be  suggested  that  they 
represent  aborted  corallites  or  attempts  at  budding.  This 
coral  belongs  to  the  order  Porifera,  which  has  been  shown 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  77 

by  the  writer7  to  have  very  pronounced  tendencies  toward 
proliferation,  and  on  the  interior  of  the  colony  these  attempts 
result  in  the  production  of  mural  pores.  Most  of  the  species 
of  Michelinia  are  hemispherical  or  spherical.  M.  favosa  is 
inclined  to  be  pyriform  in  shape,  rising  above  the  object  of 
support,  and  thus  presenting  a  rather  large  epithecal  surface. 
Manifestly  the  lower  side  of  the  corallum  is  unfavorably  situ- 
ated for  the  growth  of  corallites,  and  any  efforts  at  prolifera- 
tion on  the  part  of  the  peripheral  corallites  is  apt  to  result  in 
stunted  outgrowths.  There  is  here  a  very  close  connection 
between  restraint  of  environment  and  deficiency  of  growth 
force.  If  the  whole  corallum  is  taken  into  consideration,  the 
restraint  of  the  environment  may  be  taken  as  preventing  the 
growth  of  corallites  on  the  lower  side.  If  one  of  these  single 
stunted  corallites  is  considered,  it  may  be  said  that  the  defi- 
ciency of  growth  force  through  lack  of  nutrition  caused  its 
suppression. 

IX.    Mechanical  restraint.     (C2.) 

Among  the  factors  of  spine  genesis  mechanical  restraint  is 
probably  of  the  least  importance.  It  can  only  rarely  happen 
that  an  organism  is  forced  to  grow  a  spine  contrary  to  the 
natural  tendencies  of  normal  development.  Yet,  as  there  are 
occasional  types  of  spiniform  structures  which  can  be  best 
explained  as  due  to  the  mechanical  restraint  of  the  environ- 
ment, it  is  necessary  to  notice  them  in  order  to  make  the 
categories  of  origin  as  complete  as  possible. 

The  illustrations  will  be  taken  chiefly  from  the  Brachiopoda 
and  Trilobita.  The  recent  brachiopod  Muhlfeldtia  truncata 
is  semi-elliptical  in  outline,  and  has  a  very  short  stout  pedicle 
which  holds  the  shell  so  closely  to  the  object  of  support  that 
the  beak  is  truncated  from  abrasion  and  resorption.  In 
specimens  attached  to  a  small  branch  of  a  coral,  thus  allowing 
the  cardinal  extremities  of  the  shell  to  project  beyond  the 
object  of  support,  the  ends  of  the  hinge  are  generally  rounded. 
Specimens  growing  on  a  large  flat  surface  have  the  cardinal 
extremities  angular  or  sub-mucronate.  Similar  variations  are 


78  STUDIES  IN  EVOLUTION 

to  be  observed  in  other  living  species  of  Brachiopoda  (Cfo- 
tella,)  some  Dallina,  etc.)-  Some  of  the  extinct  genera  show 
more  highly  developed  cardinal  extremities  which  are  often 
very  characteristic  of  certain  species,  though  considerable 
variation  is  found  to  exist.  It  is  evident  that  these  elon- 
gated hinge-lines  have  arisen  from  the  mechanical  necessities 
of  a  functional  hinge,  and  their  greater  or  less  extent  is  also 
to  a  degree  dependent  upon  the  nature  of  the  object  of  sup- 
port, which  furnishes  a  stimulus  to  the  growing  ends  of  the 
hinge.  A  marked  example  is  shown  in  Spirifer  mucronatus, 
60  with  the  cardinal  angles  extended 

into    spiniform  processes  (figure   60). 

Similar  features  are  presented  by  many 

FIGURE  To.  -  Dorsal      °ther  SP6cieS  °f  Spirifer,    Orthis,  Lep- 

view  of  Spirifer  mucrona-     tcena,  Stropheodonta,  etc. 

tus;  Devonian;  showing        In  the  Trilobita  the   pygidium,  or 

spiniform  cardinal  angles.  .  .  .  . 

X  f.  (After  Hall  and  abdominal  portion,  consists  of  a  num- 
Clarke.)  ber  of  consolidated  segments,  and  the 

segments  of  the  thorax  are  successively  added  in  front  of  this 
tail  piece.  The  first  thoracic  segment  is  therefore  formed 
between  the  cephalon  and  pygidium,  and  its  form  is  mechani- 
cally in  agreement  with  the  requirements  of  the  animal  for 
bending  the  body,  and  with  the  adjacent  margins  of  the 
cephalon  and  pygidium.  In  a  way  it  may  be  said  that  the 
segment  is  moulded  by  the  adjacent  _parts.  and  may  there- 
fore take  its  form  from  the  cephalon  (figure  61),  or  from  the 
pygidium,  as  in  the  examples  following  (figures  62-65). 

During  growth  the  new  segments  are  added  in  front  of  the 
anal  segment,  so  that  after  the  number  of  abdominal  seg- 
ff ]J[fijfc4  ments  is  complete  the  thorax  is  increased  by  the  successive 
/r  uL    .Addition  of  what  in  earlier  moults  were  pygidial  segments. 
*  /By  this  means  the  pygidium  generally  controls  or  determines 
**¥         J&8  cnarac^er  °f  "the  segments  of  the  thorax.     If  the  pleura 
/^xj^of  the  pygidium  are  extended  into  spiniform  processes,  the 
jjljy \  pleural  ends  of  the  segments  are  also  spiniform;  as  in  Lichas 
(figure  64),  Ceraurus,  Cheirurus  (figure  62),  Deiphon  (figure 
63),  Acidaspis,  Dindymene,  etc. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


79 


Likewise,  if  the  pleura  or  their  distal  ends  are  directed 
posteriorly  nearly  parallel  to  the  axis,  the  mechanical  neces- 
sities of  motion  require  that  the  portions  of  the  free  segments 
pointing  backward  should  be  free,  thus  making  the  ends  of 


61 


62 


63 


64 


65 


FIGURE  61.  —  lUoenus  (Octillanus)  Hisingeri,  Ordovician,  Bohemia ;  a  trilobite ^ 
showing  spiniform  pleural  extremities  of  first  thoracic  segment,   corresponding 
to  the  genal  spines  of  the  cephalon.     X  f-     (After  Barrande.4)  /?(*££,  0^  /^ 

FIGURE  62.  —  Cheirurus  msignis,  Silurian,  Bohemia ;  pygidium  and  six. 
thoracic  segments.  X  f.  (After  Barrande.) 

FIGURE  63.  —  Deiphon  Forbesi,  Silurian,  Bohemia;   entire  specimen;  show-     f^fT^ 
ing  spiniform  pleura  of  segments  corresponding  in   direction  to  those  of  the         (/ 
pygidium.     (After  Barrande.) 

FIGURE  64. — Lichas  scabra,  Silurian,  Bohemia;  pygidium,  with  three 
thoracic  segments  ;  showing  spiniform  ends  of  pleura.  X  f •  (After  Barrande.) 

FIGURE  65.  —  Paradoxides  spinosus,  Cambrian,  Bohemia.;  pygidium  and  six    ^  j  Jfr 

free  segments.     X/f-    {After  Barrande.)    ^/>££^ 

ju£&JC*^ji%^   ^^Ski^^J  ^^^^aJ^ff^  CS--#*&* 

ie  tnoracic  pleura  generally  appear  as  retrally  curved  spini- 
form extensions.  Extreme  examples  of  retrally  directed 
pleura  accompanied  by  small  pygidia  are  shown  in  Para- 
doxides  (figure  65),  Holmia,  Olenellus,  Elliptocephala,  etc. 


80  STUDIES  IN  EVOLUTION 

Genera  having  the  ends  only  of  the  pleura  directed  backward 
are  generally  less  inclined  to  form  spiniform  terminations. 
In  contrast  with  these  it  is  found  that  all  the  Trilobita 
having  the  pleura  directed  outward,  and  with  entire  pygidial 
margins,  do  not  ordinarily  develop  long  pleural  spines;  as 
Asaphus,  Illcenus,  Agnostus,  Phacops,  Calymmene,  etc. 

The  examples  of  the  caterpillars  of  moths  belonging  to  the 
Schizurse,  described  by  Packard  64  as  mimicking  the  serrations 
of  the  leaves  upon  which  they  feed,  have  previously  been 
noticed  in  this  essay,  under  the  head  of  mimetic  influences. 
The  initial  cause  of  the  spines  may  possibly  be  explained  as 
in  part  due  to  the  mechanical  conditions.  During  their  early 
existence  the  larvae  feed  on  the  lower  side  of  the  leaves,  and 
have  no  spines.  Later  they  feed  on  the  edges  of  the  leaves, 
at  the  same  time  acquire  dorsal  spines.  The  conforma- 


^  rf  /tion  of  the  animal  to  the  serrated  edge  of  the  leaf  would 
^  produce  corresponding  elevations  and  depressions  on  the  back. 
The  location  of  these  would  be  fairly  constant  from  the  habit 
of  the  animal  of  feeding  chiefly  between  the  denser  leaf  veins 
which  determine  and  terminate  the  serrations.  The  raised 
parts  of  the  animal  would  receive  the  greatest  amount  of 
stimuli,  and  at  these  points  spines  would  naturally  appear. 

The  processes  producing  the  spines  noticed  in  this  category 
/(IX)  are  classed  with  others  under  decrescence,  for  the  reason 
that  the  growth  is  restrained  or  controlled  by  mechanical 
necessities.     If  the  restraint  were  absent,  it  is  probable  that 
a  more  expansive  growth  would  take   place  or   that   other 
structures  would  be  correspondingly  benefited. 
f 

X.    Disuse.     (C3,  D2.) 

In  causing  the  reduction  or  atrophy  of  an  organ,  the  effects 
of  disuse  have  generally  been  recognized  by  most  observers. 
In  this  way  the  origin  of  many  of  the  so-called  "  rudimentary 
organs  "  has  been  satisfactorily  explained  by  Darwin  14  and 
others.  Two  classes  of  structures  are  evidently  comprised 
within  the  common  definition  of  rudimentary  organs  ;  namely, 
nascent  and  vestigial  organs. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  81 

Nascent  structures  indicate  the  beginnings  or  initial  stages 
of  organs,  while  vestigial  structures  are  the  remnants  left 
after  the  functional  suppression  of  organs.  The  suppression 
is  usually  caused  by  unfavorable  conditions  or  by  disuse, 
which  produces  either  a  retardation  of  growth  or  a  retrogres- 
sive development.  In  both  cases  the  results  are  similar. 
By  retardation  an  organ  is  prevented  or  restrained  from  func- 
tional development  and  is  therefore  useless  as  a  normal  organ. 
By  retrogression  an  organ  gradually  reverts  to  an  initial  type, 
loses  its  function,  and  becomes  a  vestigial  structure.  In 
most  instances  a  change  of  food  or  habit  or  the  substitution 
of  a  new  and  functionally  higher  structure  causes  the  disuse 
of  some  organ  which  under  previous  conditions  was  of 
use  to  the  animal. 

Nascent  structures,  or  the  beginnings  of  organs,  are  gen- 
erally made  up  of  active  tissues  that  only  require  stimulus 
and  nutrition  to  perfect  their  function.  On  the  other  hand 
suppressed  or  vestigial  structures  are  composed  of  compara- 
tively inert  tissue,  and  are  in  consequence  largely  made  of 
the  mechanical  elements  of  secretion  of  the  organism.  It 
may  therefore  be  considered  that  true  rudimentary  or  nascent 
organs  are  potentially  active,  and  suppressed  structures  are 
inert.  It  is  with  the  latter  class,  the  inert,  that  a  study  of 
spine  genesis  by  atrophy  is  chiefly  concerned. 

The  gradual  loss  of  function  through  disuse,  and  the  con- 
sequent loss  of  nutrition  with  the  concomitant  rapid  decres- 
cence  of  active  tissues,  bring  about  a  change  in  the  ratio  of 
active  and  inert  structures.  The  progression  of  this  process 
naturally  results  in  the  production  of  a  structure  having  a 
maximum  of  inert  or  mechanical  tissues  and  a  minimum  of 
active  constituents.  Moreover,  it  has  already  been  shown 
that  the  axial  elements  are  the  most  persistent,  and  therefore 
the  last  to  disappear;  also  that  the  peripheral  appendages 
and  outgrowths  of  any  organ  first  show  the  action  of  decres- 
cence.  Evidently  the  conditions  here  described  are  favorable 
for  the  production  of  spines  out  of  an  organ  primarily  possess- 
ing distinct  active  functions.  The  axis  of  an  organ  gives 

6 


82  STUDIES  IN  EVOLUTION 

the  necessary  form,  and  the  hard  tissue  the  structure,  so  that 
the  whole  will  conform  to  the  definition  of  a  spine  given  early 
in  this  paper;  namely,  a  stiff,  sharp-pointed  process. 

The  restraint   of  the   environment  was   found   to  be  one 
*  (cause  for  decrescence  of  organs.     Another,  which  is  properly 
the   subject   matter  of   the  present  section,  is  disuse;   and 
lastly,  it  will  be  seen  how  the  deficiency  of  growth  force  may 
bring  a  similar  suppression  of  structures. 

There  is  considerable  difficulty  in  selecting  particular 
examples  which  will  conform  clearly  to  the  strict  require- 
ments of  these  three  categories.  In  a  certain  sense  some  of 
the  examples  of  spines  produced  by  decrescence  may  belong 
to  more  than  one  category.  However,  it  does  not  prevent  the 
acceptance  of  any  one  of  the  three  as  primary  causes.  Thus 
it  may  be  urged  that  disuse  has  caused  the  atrophy  of  leaves 
into  spines  among  many  desert  plants,  or  produced  a  similar 
reduction  of  the  limbs  in  a  Python.  While  this  may  be  true 
from  one  point  of  view,  yet  the  manifest  unfavorableness  of 
the  environment  in  both  seems  to  be  a  sufficient  reason  for 
making  it  the  primary  factor.  On  the  other  hand  many 
parasites  showing  similar  atrophies  are  not  dependent  upon  a 
large  number  of  active  organs  for  their  food  and  maintenance. 
After  finding  a  host  an  abundance  of  food  is  at  hand,  and 
the  environment  may  be  considered  a  favorable  one.  All  the 
organs,  except  those  of  nutrition  and  reproduction,  then 
become  more  or  less  useless  and  dwindle  away,  leaving 
vestigial  organs  or  disappearing  altogether.  Furthermore, 
a  change  of  habit,  as  from  climbing  to  flying,  will  necessarily 
cause  the  atrophy  of  some  of  the  structures  used  for  climbing 
and  the  hypertrophy  of  others  for  flying. 

Most  of  the  examples  illustrating  the  production  of  a  spine 
through  the  atrophy  of  an  organ  by  disuse  are  to  be  found  in 
the  legs  and  digits  of  animals.  The  process  bears  consider- 
able resemblance  to  the  formation  of  spines  on  many  plants 
by  the  suppression  of  leaves,  branches,  etc.  They  will  be 
noticed  here,  although  properly  these  vestigial  structures 
among  animals  are  more  strictly  of  the  nature  of  claws,  or,  at 
the  most,  spurs. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  83 

Many  parasitic  plants,  especially  among  the  Balanophorese, 
are  reduced  to  a  simple  stem  bearing  the  inflorescence.  The 
leaves  are  represented  by  scales  which  are  often  spiniform, 
though  seldom  of  sufficient  stiffness  to  entitle  them  to  be 
called  spines.  In  desert  plants,  many  of  which  have  a  simi- 
lar type  of  growth,  the  hardening  of  the  mechanical  tissues 
by  the  effects  of  drought  has  converted  similar  leaf  structures 
into  spines,  while  the  parasitic  plants  are  not  normally  sub- 
jected to  such  continuous  dryness  and  extreme  heat,  and 
therefore  the  mechanical  tissues  seldom  become  hardened. 

Parasitic  animals,  especially  among  the  Crustacea  and 
insects,  often  show  a  reduction  in  the  number  of  joints  in 
the  legs,  and  even  in  the  number  of  limbs  themselves.  The 
terminal  claws  generally  persist,  and  are  sometimes  longer 
than  the  rest  of  the  leg ;  as  in  the  Itch-mite  Sarcoptes  Scabiei, 
and  in  the  female  of  the  parasitic  copepod  Lernceascus  nema- 
toxys  (figure  66). 

Among  many  aquatic  Crustacea  and  limuloids,  the  special- 
ization and  segregation  of  the*  ambulatory  and  swimming 
appendages  toward  the  head  or  anterior  regions  of  the  body 
have  produced  a  corresponding  suppression  of  appendages  on 
or  near  the  extremity  of  the  abdomen.  This  statement  of 
fact  is  the  basis  of  the  principle  of  cephalization  of  Dana,12 
who  applies  it  especially  to  the  Crustacea,  as  follows :  "  There 
is  in  general,  with  the  rising  grade,  an  abbreviation  relatively 
of  the  abdomen,  an  abbreviation  also  of  the  cephalothorax  and 
of  the  antennsB  and  other  cephalic  organs,  and  a  compacting  of 
the  structure  before  and  behind;  a  change  in  the  abdomen 
from  an  organ  of  great  size  and  power  and  chief  reliance  in 
locomotion,  to  one  of  diminutive  size  and  no  locomotive 
power."  Audouin's  law  that  among  the  Articulata  one  part]' 
is  developed  at  the  expense  of  another  may  be  also  noticed 
here  as  affording  a  further  explanation  of  the  suppression  of 
the  posterior  appendages  correlative  with  the  greater  develop- 
ment of  the  parts  anterior  to  them.  In  a  Crustacean  using 
its  tail  for  propulsion,  as  the  Lobster  {Homarus),  the  telson 
is  broad  and  flat,  and  the  adjacent  segment  has  a  similar 


84  STUDIES  IN  EVOLUTION 

development  of  the  appendages.  In  other  forms,  as  the 
Horse-shoe  Crab  (Limulus)  and  the  Phyllocarida,  the  tail 
is  not  used  for  propulsion,  and  at  best  serves  chiefly  as  a 
rudder,  while  some  of  the  legs  on  the  anterior  part  of  the 
abdomen  or  on  the  thorax  are  large  and  strong  and  are  often 
provided  with  paddles.  These  groups,  the  limuloids  and 
Phyllocarida,  show  a  greater  or  less  suppression  of  the  last 
abdominal  appendages,  and  in  many  genera  the  body  termi- 
nates in  a  spiniform  telson  or  tail  spine.  The  process  of 
suppression  may  or  may  not  result  in  a  spine.  In  the  crabs 
the  abbreviated  abdomen  is  folded  under  the  cephalothorax, 
and  in  Lepidurus  and  Pterygotus  the  telson  is  a  scale  or 
plate-like  organ.  For  the  most  part,  however,  the  abbrevia- 
tion of  the  abdomen  and  the  suppression  of  its  appendages 
have  reduced  the  telson  to  a  spine;  as  in  Limulus  (figure  67), 
Eurypterus,  Stylonurus,  and  PrestwicTiia  among  limuloids, 
and  Olenellus  among  the  Trilobita.  In  addition  to  a  telson 
spine,  the  Phyllocarida  have  two  lateral  spiniform  cercopods, 
the  three  spines  together  constituting  the  post-abdomen ;  as 
in  Ceratiocaris,  Echinocaris  (figure  68),  Mesothyra,  etc. 

Although  the  last  abdominal  segments  of  the  Horse-shoe 
Crab  have  lost  their  appendages  and  show  evidences  of 
suppression,  yet  the  tail  spine  is  a  large  and  useful  organ, 
for  it  is  of  just  the  proper  length  to  enable  the  animal  to 
right  itself  after  being  overturned,  which  it  is  unable  to 
do  with  its  feet  alone.  The  process  of  natural  selection  has 
doubtless  in  this  way  contributed  to  the  development  and 
retention  of  the  long  spine.  This  use  cannot  be  ascribed 
to  the  tail  spines  of  the  Phyllocarida,  though  they  evidently 
were  important  aids  in  directing  movement,  and  also  offered 
some  degree  of  protection. 

The  terminal  claws  on  the  phalanges  of  the  wings  of  some 
birds  are  nearly  all  that  remain  of  the  external  fingers  or 
digits.  In  the  Hoactzin  of  South  America  (Opisthocomus 
cristatus)  the  young  bird  has  a  thumb  and  index  finger,  both 
provided  with  claws,  and  climbs  about  much  like  a  quad- 
ruped, using  its  feet,  fingered  wings,  and  beak.  According 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES 


85 


to  Lucas,43  a  rapid  change  "takes  place  in  the  fore  limb  dur- 
ing the  growth  of  the  bird,  by  which  the  hand  of  the  nestling, 
with  its  well-developed,  well-clawed  fingers,  becomes  the 
clawless  wing  of  the  old  bird  with  its  abortive  outer  finger." 
Similar  claws  or  spurs  occur  on  a  number  of  other  birds, 
some  having  functional  wings,  as  in  the  example  just  de- 
scribed, and  others  having  only  vestiges  of  wings,  as  in  the 
Wingless  Bird  of  New  Zealand  (Apteryx,  figure  69). 


66 


67 


FIGURE  66.  —  Female  of  LernceascuS  nematoxys,  a  parasitic  copepod  ;  showing 
suppression  of  limbs.  Enlarged.  (After  Glaus.) 

FIGURE  67. —  Horse-shoe  Crab  (Limulus  polyphemus) ;  showing  telson  spine 
and  abbreviated  abdomen.  Reduced. 

FIGURE  68.  —  A  Devonian  phyllocarid  (Echinocaris  socialis) ;  showing  spiniform 
telson  and  cercopods. 

FIGURE  69.  —  Wing  of  Apteryx  australis,     X  i-     (After  Romanes.) 

FIGURE  70.  —  Skeleton  of  right  fore  limb  of  the  Jurassic  Dinosaur  Iguanodon 
bernissartensis  ;  showing  partially  suppressed  first  digit.  X  -fa.  (After  Dollo.16) 

Another  example  may  be  taken  from  the  Dinosaurian  Rep- 
tiles. The  Jurassic  genus  Iguanodon,  from  England  and 
Belgium,  belongs  to  a  group  (Ornithopoda)  in  which  the 
number  of  functional  digits  varies  from  three  to  five  in  the 
manus,  and  from  three  to  four  in  the  hind  foot.  In  this 
genus  the  hind  foot  had  three  functional  toes,  representing 
the  second,  third,  and  fourth  of  a  normal  pentadactyl  foot. 


86  STUDIES  IN  EVOLUTION 

The  first  is  represented  by  a  slender  tarsal  bone  alone, 
while  the  fifth  is  completely  suppressed.  The  manus,  or 
fore  foot,  of  this  animal  shows  the  second,  third,  fourth,  and 
fifth  digits  of  functional  importance  as  digits,  while  the  first 
is  shortened  and  atrophied  to  the  condition  of  a  stout  spur, 
standing  out  at  right  angles  to  the  axis  of  the  leg,  as  shown 
in  figure  70.  The  fore  legs  of  Iguanodon  and  others  of  the 
same  order  were  short,  and  apparently  used  more  for  prehen- 
sion than  locomotion,  and  in  Iguanodon  the  suppression  of  the 
pollex,  or  thumb,  into  a  spur  doubtless  provided  the  animal 
with  a  powerful  weapon.  Here  is  seen  the  suppression  of  a 
digit  by  loss  of  normal  function,  resulting  in  a  protective 
structure  of  considerable  value. 


XI.  Intrinsic  suppression  of  structures  and  functions.     (Di.) 

The  most  obvious  and  direct  relationship  between  an  un- 
favorable environment  and  the  suppression  of  structures  to 
form  spines  was  afforded  by  desert  plants.  In  illustration  of 
the  intrinsic  suppression  of  structures  by  deficiency  of  growth 
force,  the  vegetable  kingdom  again  seems  to  offer  the  clearest 
evidences  of  a  like  relation  between  cause  and  effect.  Instead, 
however,  of  taking  an  unfavorable  environment,  in  the  pres- 
ent instance  a  favorable  environment  must  be  assumed,  and 
then  a  type  which  expresses  in  various  ways  its  deficiency  of 
growth  force  must  be  sought. 

In  the  desert  plants  it  was  found  that  no  single  family 
exclusively  constituted  the  desert  flora,  but  that  a  consider- 
able variety  of  types  was  present,  and  that  some  of  these 
belonged  to  perfectly  normal  families  commonly  living  under 
ordinary  favorable  conditions.  Moreover,  it  was  evident  that 
there  were  certain  types  of  form  and  habits  of  growth  which 
were  especially  characteristic  of  plants  living  in  desert  or 
similar  unfavorable  regions.  Therefore,  to  illustrate  clearly 
intrinsic  restraint  or  suppression  of  structures  it  will  be 
necessary  to  take  an  environment  which,  in  most  re- 
spects, may  be  considered  as  favorable,  and  also  a  type  of 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  87 

plant  life  presenting   evidences  of  a  deficiency  of  growth 
force. 

The  great  groups  of  plants  commonly  known  as  brambles 
and  climbing  plants  appear  to  meet  most  of  the  requirements. 
They  abound  in  regions  where  the  greatest  luxuriance  of 
vegetation  is  found,  and  are  therefore  chiefly  characteristic 
of  the  tropics.  Kerner38  estimates  that  there  are  two  thou- 
sand species  of  the  true  climbing  plants  in  the  torrid  zone, 
and  about  two  hundred  in  temperate  regions.  Tropical 
America  has  the  largest  number  of  species,  the  flora  of  Brazil 
and  the  Antilles  being  especially  rich.  In  the  sombre  depths 
of  the  tropical  forest  the  climbing  plants,  or  "lianes,"  are 
not  so  abundant  as  in  the  open  glades  and  along  the  edge 
of  the  forest,  where  the  amount  of  light  is  greater  and  the 
conditions  of  existence  are  more  favorable.  As  far  as  rich- 
ness of  soil,  amount  of  light,  and  degree  of  temperature  are 
concerned,  it  must  be  admitted  that  their  environment  is  as 
favorable  as  that  of  any  of  the  associated  plants  having  dif- 
ferent  habits  of  growth.  The  difference  between  the  strong 
and  erect  plants  and  the  comparatively  weak  and  climbing  Tj^t/w^ 
forms  is  therefore  not  an  extraneous  one.  It  resides  within  .>  A 
the  plant  structures  themselves,  and  is  an  intrinsic  character/  / 
or  an  expression  of  hereditary  vital  forces. 

The  law  of  recapitulation  demands  that  each  individual 
during  its  development  shall  pass  through  an  epitome  or 
recapitulation  of  its  ancestral  history.  In  view  of  the  fact 
that  the  young  seedlings  of  climbing  plants  and  brambles 
have  the  erect  form  and  proportions  of  normal  erect  foliage 
stems,  it  is  safe  to  infer  that  they  have  been  derived  from 
erect  forms.  Further  evidence  is  afforded  from  the  absence 
of  climbing  plants  in  the  earlier  terrestrial  floras.  It  is 
obvious,  therefore,  that  they  have  been  developed  out  of 
erect  forms  by  a  process  of  degradation. 

The  next  striking  feature  to  be  noticed  in  climbing  plants 
is  their  extreme  slenderness,  due  to  the  general  suppression 
of  the  plant  body.  They  may  attain  lengths  not  reached  by 
the  highest  trees,  and  yet  the  diameter  of  the  trunk  is  but 


88  STUDIES  IN  EVOLUTION 

a  minute  fraction  of  the  length.  The  Climbing  Palm,  or 
Ratan,  has  stems  of  great  length  and  tenuity.  It  has  been 
stated  that  stems  two  hundred  metres  long  have  been  ob- 
served having  a  uniform  thickness  of  only  from  two  to  four 
centimetres.38  The  diameter  of  such  a  stem  would  be  only 
one  or  two  ten-thousandths  of  its  length.  The  length  of 
the  internodes  is  another  conspicuous  character  in  climbing 
plants,  and  both  this  and  the  slenderness  of  the  stems  suggest 
the  results  obtained  by  growing  ordinary  plants  in  the  dark, 
where  the  conditions  are  adverse  to  increased  vitality. 

The  transfer  of  function  from  one  part  of  the  plant  to 
another,  usually  by  a  process  of  retrogression  or  degradation, 
is  also  very  common.  The  first  growth  above  the  ground  is 
a  leafy  stalk.  Later,  after  the  plant  has  attained  a  consider- 
able height,  the  lower  portion  puts  out  quantities  of  rootlets 
and  loses  its  foliage.  The  rootlets  may  be  mere  dry  threads 
or  points  of  support  for  the  stem;  or,  if  they  happen  to 
encounter  a  crevice  containing  soil,  they  develop  into  true 
absorbent  organs.  In  others  the  ends  of  the  growing  stems 
or  any  point  on  the  stems,  upon  reaching  the  earth,  may 
put  out  vigorous  roots.  These  facts  seem  to  show  a  lack  of 
|  positive  differentiation  throughout  the  plant,  which  admits 
of  the  substitution  of  a  lower  structure  for  a  higher  by  the 
suppression  of  a  higher  function. 

Lastly,  the  general  spininess  of  climbing  plants  and  bram- 
bles is  a  well-known  and  conspicuous  character.  Kerner38 
says  that  "most,  if  not  all,  plants  which  weave  into  the 
thicket  of  other  plants  are  equipped  with  barbed  spines, 
prickles  and  bristles."  These  spiniform  processes  seem  to 
fall  naturally  into  two  classes:  first,  those  produced  by  the 
suppression  of  stipules,  leaves,  petioles,  branches,  etc. ;  and 
second,  those  appearing  as  simple  eruptions  on  the  surface. 

The  suppression  of  normal  plant  organs  into  special  struc- 
tures, as  tendrils  and  claspers,  is  extremely  common,  and,  as 
already  shown,  this  process,  if  carried  far  enough  without 
complete  suppression,  will  favor  the  production  of  a  spini- 
form growth  representing  the  axial  elements  of  the  organs. 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  89 

The  classes  of  organs  thus  affected  are  practically  the  same 
as  those  in  desert  plants,  though  varying  somewhat  in  man- 
ner and  degree.  The  consolidated  type  of  plant  body  is 
naturally  absent,  for  in  this  respect  the  diffuseness  of 
climbing  plants  is  quite  antithetical.  It  does  not  seem  nec- 
essary to  give  a  long  list  of  examples  among  the  climbers, 
illustrating  the  suppression  of  organs  into  spines.  Although 

71  72  73 


FIGURE  71.  Leaf  of  Ratan  (Dcemonorops  hygrophilus).  Reduced.  (After 
Kerner.) 

FIGURE  72.  Leaf  of  Ratan  (Desmoncus  polyacanthus).  Reduced.  (After 
Kerner.) 

FIGURE  73.    Bramble  (Rubus  squarrosus).    Reduced.     (After  Kerner.) 

apparently  not  of  rare  occurrence,  spines  produced  in  this 
way  are  not  as  common  as  among  desert  plants.  Two  figures 
of  the  pinnate  leaves  of  Ratan  are  introduced  here  to  show 
the  suppression  of  a  number  of  the  terminal  leaflets  into 
spines  (figures  71,  72).  In  Machcerium  the  stipules  are 
converted  into  thorns.62  A  tropical  Bignonia  (B.  argyro- 
violacea)  has  normal  full-sized  simple  leaves,  and  suppressed 
leaves  bearing  two  opposite  leaflets  on  one  stalk,  and  ending 


90  STUDIES  IN  EVOLUTION 

"in  a  structure  which  divides  into  three  limbs,  with  pointed 
hooked  claws,  and  which  is  not  unlike  the  foot  of  a  bird 
of  prey."38 

By  far  the  greater  number  of  spines  on  climbing  plants  are 
of  the  nature  of  prickles,  and  are  not  produced  by  the  sup- 
pression of  any  particu]ar  organ  or  organs,  but  appear  usually 
without  any  very  definite  order.  They  represent  outgrowths 
of  the  superficial  layers,  and  hypertrophied  plant  hairs,  or 
trichomes.  The  cause  of  these  cortical  eruptions  is  not  clear, 
although  they  seem  to  be  intimately  connected  with  the  gen- 
eral suppression  of  the  plant  body.  They  are  therefore  a 
secondary  and  not  a  direct  result  of  suppression.  Bailey2 
asserts  that  "probably  the  greater  number  of  spinous  pro- 
cesses will  be  found  to  be  the  residua  following  the  contraction 
of  the  plant  body."  This  connection  is  very  apparent  in  the 
consideration  of  the  suppression  or  contraction  of  various 
plant  organs,  but  is  less  obvious  when  applied  to  the  surface 
of  the  whole  plant,  though  doubtless  it  is  the  true  explana- 
tion. In  continuation  of  this  idea  it  may  be  suggested  that 
the  prickles  represent  aborted  attempts  on  the  part  of  the 
plant,  through  hereditary  influences,  to  recover  its  former 
normal  proportions.  Or  they  may  exhibit  the  action  of  the 
law  of  repetition  acting  in  an  organism  where  the  initial 
cause  of  spine  production  is  the  intrinsic  suppression  of  such 
structures  as  leaves,  petioles,  stipules,  etc.  The  subsequent 
repetition  of  spines  on  other  parts  of  the  organism  results  in 
a  series  of  homoplastic  spines  which  are  not  homologous  with 
those  first  formed. 

The  prickles  on  climbing  plants  and  brambles  may  often 
serve  for  purposes  of  protection  (D3),  and  enable  the  plant  to 
cling  to  a  support,  but  these  utilitarian  properties  cannot  be 
considered  as  an  initial  cause.  Natural  selection,  also,  prob- 
ably has  fostered  the  development  of  certain  types  of  spiny 
climbers  and  the  production  of  adaptive  characters.  Never- 
theless, in  studying  these  forms,  it  is  necessary  to  revert  to 
the  original  consideration  of  the  localized  suppression  of 
normal  plant  structures,  and  to  the  general  suppression  of 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  91 

the  plant  body  as  affording  a  more  primary  conception  of  the 
causes  and  modes  of  spine  growth  among  climbing  plants. 

In  many  cases  of  retrogressive  series  of  animals  there 
seems  to  be  a  close  parallelism  with  some  of  the  characters 
observed  among  the  climbing  plants.  If  the  Ammonite  fam- 
ily during  the  Cretaceous,  or  near  the  close  of  the  Mesozoic, 
is  taken  as  an  example,  it  cannot  be  said  that  the  environ- 
ment of  these  old-age  or  pathologic  series  is  unfavorable  in 
respect  to  food,  temperature,  etc.,  for  with  them  are  assor 
ciated  many  vigorous  progressive  series  of  other  organisms. 
Neither  can  it  be  said  that  in  many  cases  the  animals  perished 
on  account  of  over-specialization^  though  this  was  evidently 
the  cause_of  the  extinction  of  a  large  number.  The  return 
to  a  condition  of  second  cTTildhdod  in  old  age  cannot 
called  a  progressive  specialization,  since  it  clearly  points  to  aT 
deficiency  of  growth  force. 

Old-age  types,  or  phylogerontic  forms,  among  animals  may 
show  the  same  attenuation  or  suppression  of  the  body  as  do 
climbing  plants.     Thus  Baculites,  considered  by  Hyatt  as  a 
typical  phylogerontic  type,  has  a  very  attenuate  shell,  and 
some  species,   after  attaining  a  certain   diameter,   cease   to 
increase   in   any   direction   except   length.     On   account   of 
being   a   chambered   shell,  it   is   manifest   that  the  growth 
of  the  animal  must  have  practically  ceased,  while  its  secretive  ^~fj 
activities  were  continued  and  confined  largely  to  lengthening^^ 
the  shell.     Other  related  genera  of  Cephalopoda  show  a  simi-/&  /** 
lar  attenuation  of  the  shell,  evincing  a  stoppage  of  growth 
in  the  animal.     Among  the  Mollusca  it  seems  quite  likely 
that  attenuation  of  form  often  accompanies  decreased  growth 
power. 

The  pathologic  varieties  of  the  Steinheim  Planorbis,  as 
described  by  Hyatt,35  or  of  the  recent  Planorbis  complanatus 
described  by  Fire*,57  are  further  illustrations  of  this  attenua- 
tion accompanying  the  uncoiling  of  the  shell.  The  sedentary 
Magilus,  immersed  in  its  coral  host,  is  also  an  example,  for 
not  only  does  the  shell  cease  to  increase  in  diameter,  but  the 
whole  interior,  except  a  small  cavity  at  the  end,  is  filled 


92  STUDIES  IN  EVOLUTION 

with  a  solid  deposit  of  lime.  Similar  examples  could  be 
multiplied  indefinitely.  Since,  however,  but  few  of  them 
are  spiniferous,  their  consideration  does  not  properly  come 
within  the  scope  of  the  present  discussion,  though,  as  is 
well  known,  some  of  the  attenuate  forms  often  enlarge  and 
contract  periodically,  such  enlargements  frequently  leaving 
prominent  laminae  or  nodes  that  are  sometimes  differentiated 
into  spines.  They  suggest  the  observations  on  growth,  senes- 
cence, and  rejuvenation,  made  by  Minot,48  who  showed  that 
in  guinea  pigs  from  a  very  early  age  the  increments  of 
growth  are  in  a  steadily  decreasing  ratio  to  the  increase 
of  weight  of  the  animal.  This  led  to  the  general  conclusion 
that  the  whole  life  of  an  individual  is  a  process  of  senescence 
or  growing  old. 

Spines  arising  by  a  real  pathologic  or  diseased  condition 
of  the  individual  can  have  little  or  no  effect  in  producing  a 
normal  spiniferous  variety  or  species.  However,  some  note 
should  be  taken  of  them,  especially  as  they  may  be  con- 
genital, and  thus  appear  through  several  generations.  In  the 
human  species  the  peculiar  skin  disease  known  as  ichthyosis 
sometimes  produces  spiniform  excrescences,  and  the  victims 
are  commonly  called  "porcupine-men."  The  most  cele- 
brated instance  was  the  Lambert  family.  Haeckel27  gives 
the  following  account  of  this  family:  "Edward  Lambert, 
born  in  1707,  was  remarkable  for  a  most  unusual  and  mon- 
strous formation  of  the  skin.  His  whole  body  was  covered 
with  a  horny  substance,  about  an  inch  thick,  which  rose  in 
the  form  of  numerous  thorn-shaped  and  scale-like  processes, 
more  than  an  inch  long.  This  monstrous  formation  of  the 
outer  skin,  or  epidermis,  was  transmitted  by  Lambert  to  his 
sons  and  grandsons,  but  not  to  his  granddaughters.  The 
transmission  in  this  instance  remained  in  the  male  line,  as 
is  often  the  case."  Other  similar  examples  are  cited  by 
Gould  and  Pyle,21  and  the  disease  is  described  as  "a  morbid 
development  of  the  papillae  and  thickening  of  the  epidermic 
lamellae." 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  93 

CATEGORIES  OF  INTERPRETATION 

Having  thus  far  examined  the  factors  governing  the  origin 
of  spines,  and  found  that  they  could  be  grouped  into  a 
number  of  distinct  categories,  it  is  now  desirable  to  interpret 
these  results,  and  endeavor  to  arrive  at  the  real  significance 
of  the  spinose  condition. 

The  two  main  generalizations  which  will  be  discussed  are, 
first,  that  spinosity  represents  the  limit  of  morphological 
variation,  and,  second,  it  indicates  the  decline  or  paracme  of 
vitality. 

Spinosity  a  Limit  to  Variation. 

A  number  of  data  have  already  been  given,  leading  to  the 
belief  that,  on  becoming  spinose,  organisms  have  reached  a 
limit  of  morphological  variation.  They  may  continue  to 
develop  more  and  more  differentiated  and  compound  spines, 
but  no  new  types  evolve  out  of  such  a  stock. 

The  subject  may  be  treated  in»  two  ways,  both  leading  to 
the  same  conclusion.  First,  the  stages  and  processes  involved 
in  the  growth  of  a  spine  itself  may  be  studied,  and  next  the 
development  of  spines  in  the  ontogenies  and  phylogenies  of 
animals  and  plants  may  be  examined. 

The  growth  of  a  spine  has  already  been  described,  and  it 
was  shown  that  this  type  of  growth  may  arise  from  speciali- 
zation of  other  ornamental  features,  such  as  nodes,  ridges, 
and  lamellae,  and  also  from  the  decadence  of  leaves,  legs,  etc. 
These  observations  and  numberless  others  which  could  be 
made,  will  be  sufficient  to  show  that  almost  any  kind  of 
superficial  structure,  as  knobs,  tubercles,  ridges,  laminae, 
reticulations,  etc.,  has  by  differential  growth  been  changed 
into  spines;  also,  that  organs  of  various  kinds,  as  legs, 
branches,  leaves,  etc.,  have  by  atrophy  been  reduced  to 
spines.  In  each  case  the  parts  in  their  development  pass 
through  the  various  intermediate  stages,  and  clearly  show 
that  the  spine  is  a  result  and  not  a  mean.  Moreover,  none 
of  these  structures  or  organs  are  developed  through  the 


94  STUDIES  IN  EVOLUTION 

contrary  process ;  namely,  that  of  beginning  with  spines  and 
passing  through  stages  corresponding  to  laminae,  ridges, 
tubercles,  etc.  The  spine  is  the  limit,  and  out  of  it  no 
further  structure  is  formed. 

It  is  necessary  to  make  some  mention  here  of  the  movable 
spines  of  Echinodermata,  which  appear  to  form  an  exception 
to  the  foregoing  statements.  There  seems  to  be  no  doubt 
that  the  fixed  and  movable  spines,  the  pedicellarise,  the 
paxillse,  and  the  spheridia,  are  homologous  structures,  and 
that  all  begin  as  spiniform  skeletal  outgrowths,  which  by 
subsequent  growth  and  modification  produce  the  structures 
mentioned  (Agassiz 1).  The  echinoderm  skeleton,  including 
spines,  etc.,  is  deposited  in  the  midst  of  living  tissue,  and  in 
the  case  of  the  spines  cannot  be  directly  correlated  with  the 
spines  of  other  classes  of  organisms,  which  are  either  very 
deficient  in  vitality  or  are  dead  structures  as  soon  as  com- 
pleted. After  the  movable  spines  of  echinoderms  are  fully 
developed,  the  living  portion  is  often  confined  to  the  base, 
and  the  shaft  becomes  simply  a  dead  structure  upon  which 
encrusting  organisms  may  find  lodgment,  a  condition  seldom 
occurring  in  the  living  spines.  These  finished  spines  never 
develop  into  anything  else,  and  are  the  structures  which 
conform  to  the  present  discussion.  The  embryonic  condition 
of  the  spines  and  pedicellarise  shows  that  they  are  really  more 
internal  than  external  structures,  and  therefore  remain  under 
the  full  control  of  the  ordinary  processes  of  growth,  resorp- 
tion,  and  modification  by  living  tissues.  Furthermore,  the 
movable  spines  are  of  such  functional  importance  that  no 
close  homologies  can  be  made  with  ordinary  spines  found  in 
other  classes  of  organisms. 

In  tracing  the  ontogeny  of  a  spinose  form,  it  has  been 
found  (pp.  18-22)  that  each  species  at  the  beginning  was 
plain  and  simple,  and  at  some  later  period  spines  were  grad- 
ually developed  according  to  a  definite  sequence  of  stages. 
Usually  after  the  maturity  of  the  organism  the  spines  reach 
their  greatest  perfection,  and  in  old  age  there  is  first  an 


ORIGIN  AND  SIGNIFICANCE   OF   SPINES  95 

over-production  or  extravagant  differentiation,  followed  by  a 
decline  of  spinous  growth,  and  ending  in  extreme  senility 
with  their  total  absence. 

There  are  abundant  reasons  for  believing  that  the  radicles 
of  groups  are  undifferentiated  and  inornate,  and  whenever  a 
class  has  had  a  long  existence  it  has  been  by  the  continuance 
of  such  radical  types  or  by  the  development  of  secondary  or 
tertiary  radicles,  which,  though  differing  in  internal  charac- 
ters, still  retain  a  primitive  simplicity  in  superficial  features. 
The  early  stages  of  ontogeny  of  any  form  should  agree  with 
the  radical  stock,  and,  as  already  noted,  these  stages  are 
simple.  Hyatt34  says  on  this  point:  "the  evidence  is  very 
strong  that  there  is  a  limit  to  the  progressive  complications 
which  may  take  place  in  any  type,  beyond  which  it  can  only 
proceed  by  reversing  the  process  and  retrograding.  At  the 
same  time,  however,  the  evidence  is  equally  strong  that  there 
are  such  things  as  types  which  remain  comparatively  simple, 
or  do  not  progress  to  the  same  degree  as  others  of  their  own 
group.  Among  Nautiloidea  an4  Ammonoidea  these  are  the 
radicle  or  generator  types.  No  case  has  yet  been  found  of 
a  highly  complicated,  specialized  type,  with  a  long  line  of 
descendants  traceable  to  it  as  the  radicle,  except  the  progres- 
sive ;  and  all  our  examples  of  radicles  are  taken  from  lower, 
simpler  forms;  and  these  radicle  types  are  longer-lived, 
more  persistent,  and  less  changeable  in  time  than  their 
descendants." 

A  few  examples  will  now  be  taken  from  the  life  histories 
of  large  groups.  In  the  Brachiopoda  the  order  Protremata, 
containing  most  of  the  spinose  forms,  has  4  genera  and  22 
species  in  the  Cambrian  of  America,  20  genera  and  173 
species  in  the  Ordovician,  and  30  genera  in  the  Silurian. 
"Then  began  a  steady  decline,  with  extinction  in  the  Car- 
boniferous of  North  America.  In  the  Triassic  of  Europe 
this  order  is  sparingly  represented  by  small  species,  and  is 
there  essentially  restricted  to  the  family  Thecidiidse,  which 
continues  to  have  living  representatives  in  the  Mediterranean 


96  STUDIES  IN  EVOLUTION 

Sea"  (Schuchert64).  The  super-family  Strophomenacea  of 
this  order  is  the  longest  lived,  and  excelled  in  amount  of 
specific  differentiation,  there  being  608  species  in  North 
America  alone  (Schuchert).  In  this  super-family  the  early 
families  and  genera  were  without  spines,  it  being  only  when 
Ohonetes  is  reached  that  the  first  spines  are  found  in  the 
order.  In  this  genus  they  are  along  the  hinge  and  seem  to 
make  up  for  the  weak  and  obsolescent  pedicle.  Greater 
spine  growth  occurs  in  the  genera  Productella  and  Productus, 
where,  in  extreme  cases,  the  surfaces  of  both  valves  are 
thickly  studded.  During  the  Carboniferous  the  spiny  Pro- 
ducti  attained  their  maximum  both  in  number,  length  of 
spines,  and  in  individual  size,  for  here  occur  the  largest 
species  of  all  Brachiopoda.  This  was  the  climax.  The 
Permian  genera  are  chiefly  degenerate  forms  (Aulosteges, 
Strophalosia),  and  with  the  close  of  the  Paleozoic  the  family 
Productidse  became  extinct.  The  order  Protremata,  to  which 
this  family  belongs,  likewise  underwent  a  rapid  decline,  and 
only  two  simple  types  continued  on  into  the  Mesozoic,  while 
but  one  declining  representative  is  living  at  the  present  time. 

Among  the  Ammonoidea  the  chief  spiny  forms  are  those 
occurring  just  before  the  final  extinction  of  the  group  and 
representing  the  beginning  of  the  decline  of  the  order 
(Oriocenu,  Toxoceras,  Ancyloceras,  Hamites,  etc.).  In  the 
Dinosaurian  Reptiles  the  great  horned  forms,  Triceratops, 
Torosaurus^  etc.,  mark  the  extinction  of  the  entire  order. 
The  great  horned  mammals  of  the  Eocene,  the  Dinocerata, 
have  left  no  descendants,  and  the  giant  Brontotheriidse,  after 
undergoing  various  horn  modifications  through  the  Miocene, 
continued  no  further. 

It  is  not  desirable,  however,  to  convey  the  impression  that 
the  spines  or  horns  are  alone  responsible  for  this  wholesale 
extinction.  It  has  been  shown  that  they  are  undoubtedly 
often  an  expression  of  extreme  specialization,  and  generally 
they  represent  the  limits  to  which  superficial  structures  may 
be  differentiated.  Although  there  may  be  other  expressions 
for  similar  conditions,  yet  the  presence  of  spines  is  one,  if  not 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES  97 

the  most  evident,  marker  of  the  attainment  of  these  limits. 
The  presence  of  a  spine  on  an  organ  or  part  indicates  the 
limit  of  progression  or  regression  of  that  part  or  organ.  If 
the  spinose  condition  is  general,  or  if  it  dominates  important 
functions,  it  then  indicates  the  limit  of  progression  and  regres- 
sion of  the  organism. 

Spinosity  the  Paracme  of  Vitality. 

The  physiological  interpretation  of  spinosity  is  a  correlative 
of  the  morphological  aspect  of  the  same  condition,  and,  as  it 
was  found  that  spinosity  was  a  limit  to  morphological  progress 
or  regress,  it  will  now  be  shown  that  it  also  indicates  the  par- 
acme  or  decline  of  physiological  progress.  Both  inferences 
are  drawn  from  the  individual  or  ontogenetic  standpoint,  as 
well  as  from  the  racial  or  phylogenetic. 

In  the  spinose  individual  the  decline  of  vitality  has  been 
studied  by  Geddes20  in  thorny  plants.  He  concludes  that 
they  show  a  "  gradual  death  from  point  backwards  (i.  e.  ebbing 
vitality}"  The  requisite  evidence  is  afforded  in  the  experi- 
ence of  gardeners  who  generally  consider  spiny  plants  as 
"  always  given  to  die  back,"  or,  as  otherwise  expressed,  they 
"  often  prune  themselves."  It  is  difficult  to  adduce  the  same 
kind  of  evidence  among  animals,  though  there  may  be  some 
degree  of  semblance  between  this  self-pruning  of  spiniferous 
plants  and  the  growth,  death,  and  shedding  of  the  antlers  of 
the  modern  Deer.  Stronger  evidence  of  the  relations  of 
spinosity  to  the  organism  is  afforded  in  the  consideration 
of  spines  as  consisting  wholly  of  the  mechanical  tissues. 
They  are  more  or  less  dead  structures  and  are  usually  with- 
out special  physiological  function.  Hence,  in  so  far  as  the 
whole  or  a  part  of  an  organism  is  spinose,  it  represents  the 
ratio  between  the  mechanical  and  active  tissues,  or  between 
the  inert  and  living  structures. 

Morris49  correlates  the  mechanical  and  motor  defences  of 
animals  and  plants  in  a  manner  bearing  upon  this  subject  as 
follows :  "  If  we  examine  the  whole  range  of  the  animal 
kingdom,  we  find  every  phase  of  combination  of  mechanical 

7 


98  STUDIES  IN  EVOLUTION 

and  motor  defence,  the  motion  growing  more  sluggish  as  the 
defensive  armor  grows  more  efficient.  But  in  the  whole  king- 
dom motion  persists  as  one  of  the  defensive  agencies.  No 
animal  exists  without  some  power  of  motion,  by  whose  aid  it 
withdraws  or  otherwise  escapes  from  danger."  He  also  notes 
that  the  plant  kingdom,  with  the  exception  of  the  minute, 
swimming  forms,  possesses  no  defensive  motion,  and  that 
mechanical  defence  alone  exists.  Under  mechanical  defence 
are  included  thorns,  spines,  etc.,  together  with  chemical  appli- 
ances ;  as  in  plants  with  poisonous  or  disagreeable  juices. 
These  facts  lead  to  the  conclusion  that,  in  proportion  as 
animals  are  spinose  or  armored,  they  exhibit  a  vegetative  type 
of  structure,  and  have  retrograded. 

It  has  been  shown  elsewhere  in  this  article,  that  the  great- 
est development  of  spinose  organisms  occurs  just  after  the 
culmination  of  a  group,  and,  as  this  period  clearly  represents 
the  beginning  of  the  decline  of  the  vitality  of  the  group,  the 
spines  are  to  be  taken  as  the  visible  evidence  of  this  decadence. 
A  similar  observation  has  been  made  by  Packard,54  who  after 
passing  in  review  the  geological  development  of  the  Trilobita, 
Brachiopoda,  and  Ammonoidea,  states  that  "  these  types,  as  is 
well  known,  had  their  period  of  rise,  culmination,  and  decline, 
or  extinction,  and  the  more  spiny,  highly  ornamented,  abnor- 
mal, bizarre  forms  appeared  at  or  about  the  time  when  the 
vitality  of  the  type  was  apparently  declining." 
^  Furthermore,  it  is  now  commonly  agreed  that  all  groups 
V  have  been  most  plastic  near  their  point  of  origin,  or,  in  other 
words,  that  during  their  early  history  all  the  important  or 
Y  sf~  major  types  of  structure  have  been  developed.  Their  subse- 
quent history  reveals  the  amount  of  minor  differentiation  and 
specialization  they  have  undergone.  Apparently,  most  of  the 
early  impulses  of  growth,  whether  from  the  environment  or 
from  vital  forces,  resulted  in  physiological  changes  producing 
fundamental  variations  in  function  and  structure.  The  later 
influences  of  environment  and  growth  force  are  expressed  in 
peripheral  differentiation,  and  show  that  the  racial  or  earlier 
characters  had  become  fixed,  and  that  the  later  or  specific 


ORIGIN  AND  SIGNIFICANCE  OF  SPINES  99 

features  were  the  chief  variables.  The  stimuli  which,  during 
the  early  life  history  of  a  group,  were  expended  in  internal 
or  physiological  adjustments,  later  produce  external  differen- 
tiation, and  in  this  differentiation  spinosity  is  the  limit.  The 
presence  of  spines,  therefore,  indicates  the  fixity  of  the  primary 
physiological  characters,  together  with  the  consequent  inability 
of  the  organism  to  change  due  to  its  decreasing  vitality. 

Conclusion. 

Just  as  all  the  features  of  terrestrial  topography  are  in- 
cluded between  the  limits  of  plains  and  mountains,  and  the 
mountains  are  considered  as  the  limit  of  progressive  ac- 
cidentatiou,  so  the  spines  of  animals  or  the  monticules  and 
pinnacles  of  their  surface  may  be  considered  as  the  limits  of 
progressive  differentiation.  The  primitive  base  level,  or 
peneplain,  becomes  elevated,  and  by  erosion  is  cut  up  into 
tablelands,  mesas,  and  buttes,  with  intersecting  valleys. 
The  valleys  are  gradually  deepened,  and  the  country  becomes 
rougher  until  a  maximum  is  reached.  Then  follows  a  reduc- 
tion of  the  inequalities  of  the  surface,  and  finally,  in  old  age, 
the  smooth,  gently  rounded  outlines  of  geographic  infancy 
again  appear.  So  in  organisms  the  smooth  rounded  embryo 
or  larval  form  progressively  acquires  more  and  more  pro- 
nounced and  highly  differentiated  characters  through  vouth 
and  maturity.  In  old  age  it  blossoms  out  with  a  galaxy  of 
spines,  and  with  further  decadence  produces  extravagant 
vagaries  of  spines,  but  in  extreme  senility  comes  the  second 
childhood,  with  its  simple  growth  and  the  last  feeble  infantile 
exhibit  of  vital  power. 

The  history  of  a  group  of  animals  is  the  same.  The  first 
species  are  small  and  unornamented.  They  increase  in  size, 
complexity,  and  diversity,  until  the  culmination,  when  most 
of  the  spinose  forms  begin  to  appear.  During  the  decline 
extravagant  types  are  apt  to  develop,  and  if  the  end  is  not 
then  reached,  the  group  is  continued  in  the  small  and  un- 
specialized  species  which  did  not  partake  of  the  general 
tendency  to  spinous  growth. 


100 


STUDIES  IN  EVOLUTION 


Lastly,  it  must  be  determined  whether  spines  are  really 
hereditable  characters,  and  therefore  can  be  used  in  studying 
the  phylogenies  of  groups.  No  one  has  yet  been  able  to 
show  any  type  or  set  of  characters  which  cannot  be  trans- 
mitted from  parent  to  offspring.  Hyatt 34  says :  "  Every- 


74 


Ontogeny 
stages. 

Ontogeny 
condition. 

Phylogeny 
stages. 

Phylogeny 
condition. 

Chro- 
nol- 
ogy- 

Old  age  or 
gerontic 

Paraplasis 

Phylogerontic 

Paracme 

5 

Adult  or 
ephebic 

Metaplasis 

Phylephebic 

Acme 

4 

Immature 
or  neanic 

Anaplasis 

Phyloneanic 

Epacme 

3 

Larval  or 
nepionic 

Anaplasis 

Phylonepionic 

Epacme 

2 

Embryonic 

Anaplasis 

Phylembryonic 

Epacme 

1 

FIGURE  74.  —  Diagram  and  table ;  showing  correlation  of  stages  and  condi- 
tions of  development  in  the  spinose  individual,  in  its  ancestry,  and  in  time. 

thing  is  inherited  or  inheritable,  so  far  as  can  be  judged  by 
the  behavior  of  characteristics."  Furthermore,  in  a  review 
of  animal  life,  extinct  and  living,  no  one  can  fail  to  be  im- 
pressed with  the  fact  that  especially  near  the  close  of  the 
life  history  of  a  group,  or  in  a  series  of  highly  specialized 
forms,  spinose  characters  are  often  considered  as  of  supra- 


ORIGIN  AND  SIGNIFICANCE   OF  SPINES          101 

varietal  value,  and  are  rated  of  specific,  generic,  and  some- 
times of  family  rank,  or  even  higher.  They;hay^;^yefore{ 
acquired  a  fixed  importance  in  these  special  groups,  and  ar<e 
recognized  in  the  same  categories  with  ^pJiysroiogiyaL  js^J 
structural  characters.  The  differences  which  appear  at  an 
early  period  in  higher  genera  are  the  bases  of  distinction 
among  lower  genera.  If  the  spines  or  other  similar  fea- 
tures do  not  make  their  appearance  in  an  individual  until 
a  late  adolescent  stage,  they  are  usually  of  negative  value  in 
a  scheme  of  classification.  This  agrees  with  the  general 
principle  recently  suggested  by  Harris,32  that  when  the 
main  features  of  the  ornament  (=  spines,  etc.)  are  fore- 
shadowed in  the  larval  and  early  adolescent  stages,  they  are 
to  be  regarded  as  of  taxonomic  value. 

The  preceding  diagram  illustrates  the  previous  statements, 
and  shows  the  correlation  between  the  stages  and  conditions 
of  growth  in  the  ontogeny  of  a  spinose  individual,  with  its 
phylogeny,  and  also  the  chronology  of  groups  containing 
spinose  forms.  The  numbers  indicating  chronology  simply 
refer  to  successive  periods  of  time.  In  particular  cases  they 
may  be  long  geologic  ages ;  as  Cambrian,  Ordovician,  Silurian, 
Devonian,  and  Carboniferous,  or  in  other  instances  they  may 
represent  much  shorter  periods. 

From  the  study  of  the  ontogenies  of  spinose  forms,  it  has 
already  been  ascertained  that  they  were  simple  and  inornate 
during  their  young  stages ;  and  from  the  phylogenies  of  the 
same  and  similar  forms,  it  was  likewise  learned  that  they 
were  all  derived  from  non-spinose  ancestors.  It  has  also 
been  shown  that  spines  represent  an  extreme  of  superficial 
differentiation  which  may  become  fixed  in  ontogeny,  and 
the  further  conclusion,  that  spinosity  represents  a  limit  to 
morphological  and  physiological  variation,  has  been  reached. 
Finally,  it  is  evident  that,  after  attaining  the  limit  of  spine 
differentiation,  spinose  organisms  leave  no  descendants,  and 
also  that  out  of  spinose  types  no  new  types  are  developed. 


102  STUDIES  IN  EVOLUTION 


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II 

STRUCTURE   AND   DEVELOPMENT 
OF   TRILOBITES 

1.  OUTLINE  OF  A  NATURAL  CLASSIFICATION  OF  THE 

TRILOBITES 

2.  THE  SYSTEMATIC  POSITION  OF  THE  TRILOBITES 

3.  THE  LARVAL  STAGES  OF  TRILOBITES 

4.  ON  THE  MODE  OF  OCCURRENCE  AND  THE  STRUC- 

TURE AND  DEVELOPMENT  OF  TRIARTHRUS  BECKI 

5.  FURTHER  OBSERVATIONS  ON  THE  VENTRAL  STRUC- 

TURE OF  TRIARTHRUS 

6.  THE  MORPHOLOGY  OF  TRIARTHRUS 

7.  STRUCTURE  AND  APPENDAGES  OF  TRINUCLEUS 


II 

STRUCTURE  AND  DEVELOPMENT   OF 
TRILOBITES 

1.   OUTLINE   OF  A  NATURAL   CLASSIFICATION 
OF   THE   TRILOBITES* 

(PLATE  H) 

INTRODUCTION 

WITH  the  possible  exception  of  the  barnacles,  no  group  of 
arthropods  has  received  more  varied  treatment  by  specialists 
than  the  trilobites.  This  taxonomic  uncertainty  has  been 
due  mainly  to  a  lack  of  knowledge  of  the  structure,  and  to 
certain  real  or  fancied  resemblances  to  Limulus. 

The  early  references  of  trilobites  to  the  mollusks,  insects, 
and  fishes  need  not  be  noticed,  for  since  they  have  been  made 
the  subject  of  special  study  they  have  been  commonly  classed 
with  the  Crustacea  and  placed  near  the  phyllopods  by  most 
observers.  Quite  a  number  of  naturalists,  however,  still 
divorce  the  trilobites  and  limuloids  from  the  Crustacea  and 
ally  them  with  the  arachnids.  It  is  not  proposed  at  this 
time  to  discuss  the  homologies  of  Limulus,  but  the  trilobites 
show  the  clearest  evidence  of  primitive  crustacean  affinities, 
in  their  protonauplius  larval  form,  their  hypostoma  and 
metastoma,  the  five  pairs  of  cephalic  appendages,  the  slender 
jointed  antennules,  the  biramous  character  of  all  the  other 
limbs,  and  their  original  phyllopodiform  structure.  They 
differ  from  Limulus,  not  only  in  most  of  these  regards,  but 

*  Amer.  Jour.  Sci.  (4),  III,  89-106,  181-207,  pi.  iii,  1897. 


110  STUDIES  IN  EVOLUTION 

also  in  not  having  an  operculum.  From  this  and  all  other 
arthropods  they  are  distinguished  by  having  compound  eyes 
on  free-cheek  pieces  which  apparently  represent  the  pleura 
of  a  head  segment  that  is  otherwise  lost,  except  possibly  in 
some  forms  of  stalked  eyes  and  in  the  cephalic  neuromeres  of 
later  forms.  The  most  recent  discussions  as  to  the  affinities 
of  trilobites  are  to  be  found  in  the  papers  by  Bernard,7'  8» 9«  10 
Kingsley,23  Woodward,34  and  the  writer,5  where  from  the 
facts  presented  their  intimate  relationships  with  the  Crus- 
tacea follow  as  a  necessary  corollary. 

Previous  Classifications. 

The  various  schemes  of  classification  that  have  been  applied 
to  the  trilobites  since  that  of  Brongniart,11  in  1822,  have 
been  enlarged  and  revised  by  various  authors,  until  at  the 
present  time  no  particular  arrangement  of  the  families  or 
genera  can  be  said  to  be  endorsed.  The  one  which  is  gener- 
ally recognized  as  manifestly  faulty,  that  of  Barrande,3  is  the 
one  most  commonly  found  in  text-books  and  special  memoirs. 
Barrande 's  definitions  and  limitations  of  the  generic  and 
family  groups  were  natural  and  accurate,  showing  a  most 
complete  knowledge  of  generic  and  specific  values,  but  in  the 
grouping  and  arrangement  of  the  families  he  selected  char- 
acters of  secondary  rank. 

Of  all  the  investigators  who  have  attempted  any  classifica- 
tion of  the  families,  J.  W.  Salter32  seems  to  have  had  the 
clearest  insight  into  the  important  value  of  certain  characters, 
and  to  have  approached  nearest  to  a  natural  system.  In 
zoological  research  the  study  of  ontogeny  and  the  principles 
of  morphogenesis  were  then  scarcely  recognized  as  having 
any  direct  application.  It  is  quite  remarkable,  therefore, 
that  Salter,  as  early  as  1864,  should  have  singled  out,  as 
the  basis  of  his  sub-divisions,  the  characters  which  are  the 
dominant  variants  in  ontogeny. 

It  is  not  necessary  in  this  place  to  discuss  all  the  classifi- 
cations which  have  been  proposed.  Barrande 3  gives  a  com- 
plete re'sume'  down  to  1850,  and  shows  in  a  very  satisfactory 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES     111 

manner  the  weak  points  of  each,  furnishing  strong  reasons 
as  to  why  they  are  unnatural  and  therefore  untenable.  The 
underlying  principles  of  these  early  attempts  at  a  classifica- 
tion are  here  briefly  summarized:  (1)  The  first  classification 
of  trilobites  was  advanced  by  Brongniart,11  in  1822,  in  which 
all  the  forms  previously  known  as  Entomolithus  paradoxus 
were  shown  to  belong  to  five  distinct  genera.  (2)  Dalman,16 
in  1826,  made  two  groups  based  upon  the  presence  or 
absence  of  eyes.  (3)  Quenstedt,30  in  1837,  recognized  the 
number  of  thoracic  segments  and  the  structure  of  the  eyes 
as  of  the  greatest  importance.  (4)  Milne -Ed  wards,28  in 
1840,  considered  the  power  of  enrolment  as  of  prime  value. 
(5)  Goldfuss,20  in  1843,  made  three  groups,  depending  on 
the  presence  or  absence  of  eyes  and  their  structure.  (6)  Bur- 
meister,12  in  1843,  accepted  the  two  divisions  of  Milne- 
Edwards,  and  laid  stress  on  the  nature  of  the  pleura  and  the 
size  of  the  pygidium.  (7)  Emmrich's  first  scheme,17  in  1839, 
was  founded  on  the  shape  of  the  pleura,  the  presence  or 
absence  of  eyes  and  their  structure.  (8)  The  later  classifi- 
cation of  the  same  author,18  published  in  1844,  was  based  on 
whether  the  abdomen  was  composed  of  fused  or  free  seg- 
ments, and  the  minor  divisions  depended  chiefly  on  the  struc- 
ture of  the  eyes  and  the  facial  suture.  (9)  Corda,15  in  1847, 
placed  all  trilobites  in  two  groups,  one  having  an  entire 
pygidial  margin,  and  the  other  with  the  pygidium  lobed  or 
denticulate.  (10)  McCoy,25  in  1849,  took  the  presence  or 
absence  of  a  facet  on  the  pleura  for  a  divisional  character. 
As  this  is  an  indication  of  the  power  or  the  inability  of 
enrolment,  it  does  not  differ  materially  from  the  schemes  of 
Milne-Edwards  and  Burmeister. 

Zittel,35  in  a  historical  review  brought  down  to  1885,  in- 
cludes in  addition  the  schemes  of  Barrande  and  Salter,  and 
remarks  that  the  basis  of  Barrande 's  general  grouping, 
namely,  the  structure  of  the  pleura,  has  neither  a  high  phys- 
iological nor  morphological  meaning.  Both  Barrande  and 
Salter  recognize  nearly  the  same  families,  with  slight  differ- 


112  STUDIES  IN  EVOLUTION 

ences,  and  the  latter  adopts  a  division  into  two  lines,  based 
on  the  number  of  body  rings  and  the  size  of  the  pygidium. 
These  include  and  are  themselves  included  in  four  groups, 
founded  on  the  presence  and  form  of  the  facial  suture  and 
the  structure  of  the  eyes. 

Haeckel*has  recently  given  the  trilobites  their  full  value 
in  a  classification  of  the  articulates.  Although  he  has  not 
advanced  a  detailed  classification,  still  it  is  desirable  to 
review  the  ordinal  groups  which  he  proposes.  He  considers 
the  Trilobita  as  a  legion  under  the  first  class,  Aspidonia,  of 
the  Crustacea,  which  is  characterized  as  being  without  a 
nauplius  larval  form  and  as  having  a  pair  of  pre-oral  antennae. 
In  this  class  is  also  included  the  legion  Merostomata,  the 
Trilobita  being  especially  distinguished  by  the  number  and 
character  of  the  legs.  The  writer5  believes  that  it  is  now 
satisfactorily  demonstrated  that  the  protaspis,  or  early 
larval  form  of  the  trilobite,  is  a  protonauplius,  and  homol- 
ogous with  the  nauplius  of  higher  Crustacea.  Therefore 
the  Trilobita  cannot  remain  in  the  Aspidonia,  as  here 
defined. 

Haeckel  further  divides  the  Trilobita  into  two  orders,  the 
first,  the  Archiaspides  (or  Protrilobita),  and  the  second,  the 
Eutrilobita  (or  Pygidiata).  The  Archiaspides  are  represented 
by  the  families  Olenida  and  Triarthrida,  and  are  distinguished 
by  the  absence  of  a  real  pygidium,  and  by  the  complete 
homonomy  of  the  numerous  body  segments  and  their  phyllo- 
podiform  appendages.  The  families  are  themselves  distin- 
guished by  the  semi-circular  or  crescent-shaped  cephalon  and 
by  the  presence  or  absence  of  genal  spines.  The  Eutrilobita 
are  represented  by  the  families  Asaphida  and  Calymmenida, 
and  are  marked  by  the  heteronomy  of  the  body  segments  as 
expressed  in  the  functional  pygidium. 

Salter,  Burmeister,  and  Emmrich  have,  as  previously 
noticed,  attempted  to  use  the  comparative  size  and  develop- 

*  Systematische  Phylogeiiie  der  wirbellosen  Thiere  (Invertebrata).  Zweiter 
Theil,  1896. 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES    113 

ment  of  the  pygidium  for  dividing  the  trilobites  into  groups 
larger  than  families,  and  it  seems  evident  from  the  present 
state  of  knowledge  that  it  is  impossible  to  make  this  charac- 
ter of  more  than  family  or  even  generic  value.  Many  of  the 
genera  which  must  naturally  be  included  in  the  Archiaspides 
have  pygidia  that  cannot  be  said  to  be  rudimentary,  obsolete, 
or  wanting  in  function.  Even  those  genera  having  pygidia 
with  few  segments,  as  Mesonacis,  Holmia,  Paradoxides,  Sele- 
nopeltis,  Dicranurus,  Bronteus,  Harpes,  etc.,  show  in  many 
other  more  important  characters  that  they  are  highly  differ- 
entiated arid  specialized  forms,  and  that  this  feature  is  one 
expression  of  such  development.  The  futility  of  the  scheme 
is  at  once  evident  when  a  comparison  is  made  between  allied 
genera  which  present  marked  differences  in  the  size  and 
segmentation  of  the  pygidium;  as  Phacops  and  Dalmanites, 
Ceraurus  and  Encrinurus,  Calymmene  and  Homalonotus,  Harpes 
and  Trinucleus,  Mesonacis  and  Zacanthoides,  Paradoxides  and 
Dikelocephalus. 

The  last  classification  to  be  noticed  is  that  of  E.  J.  Chap- 
man,13 in  1889,  in  which  four  sub-orders  or  primary  groups  are 
proposed,  differing  considerably  from  any  previous  arrange- 
ment, and  based  upon  arbitrary  features  of  general  structure 
and  configuration,  especially  the  form  of  the  glabella,  whether 
wide,  conical,  or  enlarged.  Twenty-seven  families  are  rec- 
ognized. In  this  scheme  Trinucleus,  Ampyx,  and  ^glina 
form  one  section ;  Paradoxides  and  Acidaspis,  together  with 
Phacops  and  Encrinurus,  another;  all  under  one  sub-order. 
Omitting  the  Agnostidae,  there  are  here  considered  in  a  single 
sub-order  the  most  characteristic  representatives  of  nearly  all 
the  types  of  trilobite  structure.  Proetus,  Cyphaspis,  and 
Arethusina  fall  into  three  sections,  under  two  sub-orders, 
although  these  genera,  on  account  of  their  great  similarity 
in  essential  points,  are  placed  in  a  single  family  by  most 
authors.  A  further  analysis  of  this  classification  in  its 
broader  lines  would  be  unprofitable.  It  is  sufficient  to  state 
that  the  facts  obtained  from  the  study  of  the  ontogeny  of 

8 


114  STUDIES  IN  EVOLUTION 

any  species  are  completely  in  discordance  with  these  classifi- 
cations, and  clearly  demand  other  interpretations. 

Rank  of  the  Trilobites. 

As  to  the  rank  of  the  trilobites  in  a  classification  of  the 
Crustacea,  there  is  also  much  diversity  of  opinion.  They 
have  long  been  regarded  as  an  order,  but  any  attempt  to 
include  them  in  this  way  under  higher  groups,  such  as  the 
Entomostraca,  Malacostraca,  or  Palsoocarida,  results  in  such 
broad  generalities  and  looseness  of  definition  as  to  render 
these  divisions  of  little  value.  Even  the  Entomostraca,  as 
restricted  to  the  orders  Phyllopoda,  Ostracoda,  Copepoda, 
and  Cirripedia,  seem  heterogeneous  and  probably  polyphy- 
letic.  Milne-Edwards,27  Gegenbaur,19  Walcott,33  and  others 
have  considered  the  trilobites  as  belonging  to  a  class  of 
arthropods  intermediate  between  the  Crustacea  arid  arach- 
nids. Some  recent  authors,  as  Lang,24  have  attempted  to 
overcome  the  difficulty  by  attaching  them  as  an  appendage 
("  Anhang  ")  to  the  Crustacea.  Kingsley, 2S  on  the  other  hand, 
has  placed  them  as  a  sub-class  of  the  Crustacea,  leaving  all 
the  other  Crustacea  to  come  under  a  second  sub-class,  the 
Eucrustacea.  The  present  state  of  knowledge  of  their  struc- 
ture and  development  is  in  favor  of  giving  the  trilobites  the 
rank  of  a  sub-class,  but  for  purposes  of  comparison  and  corre- 
lation the  fullest  results  can  be  brought  out  by  recognizing 
the  old  and  well-known  sub-classes,  —  the  Entomostraca  and 
Malacostraca. 

The  following  tabular  view  of  the  leading  points  of  the 
comparative  morphology  of  the  three  sub-classes  is  intro- 
duced to  show,  first,  the  claims  of  the  Trilobita  as  an  equiv- 
alent group,  and,  second,  the  progressive  differentiation  of 
characters.  In  nearly  every  particular  the  trilobite  is  very 
primitive,  and  closely  agrees  with  the  theoretical  crusta- 
cean ancestor.  Its  affinities  are  with  both  the  other  sub- 
classes, especially  their  lower  orders,  but  its  position  is  not 
intermediate. 


NATURAL   CLASSIFICATION  OF  THE  TRILOBITES     115 


Comparative  Morphology  of  Crustacea. 


Sub-class  I.    Trilobita. 

Sub-class  II.     Entomostraca. 

Sub-class  III.     Malacostraca. 

1.   All  marine. 

Marine  and  freshwater. 

Marine  and  freshwater. 

2.   Free. 

Free,   parasitic,  and    at- 

Free and  parasitic. 

tached. 

3.   Body    longitudinally 

Various. 

Various. 

tri-regional. 

4.    Larva    a    protonau- 

Larva  almost  universally  a 

Larva  generally  a  zoe'a, 

plius. 

nauplius. 

a  nauplius  stage  being 
often  developed  before 

hatching,  except  in  Eu- 

phausia  and  Peneus. 

5.   Number  of  segments 

Number  of  segments  vari- 

Definite number  of  seg- 

variable. 

able. 

ments. 

6.    Cranidium     of     five 

Head   of  five  fused  seg- 

Head of   five   fused  seg- 

fused segments. 

ments  to  which,  rarely, 

ments  to  which  one  or 

a  thoracic  segment  is 

more,  or  all  of  the  tho- 

added. 

racic     segments     may 

unite,  forming  a  more 

or  less  complete  ceph- 

alothorax. 

7.   Ocelli  rarely  present. 

Ocelli  present  throughout 

Ocelli    absent    in    adult 

life. 

forms. 

8.   Paired  compound  ses- 
sile eyes  on  cheek  pieces 

Paired     compound     eyes 
usually  present  ;  stalked 

Paired     compound    eyes 
usually  present;  stalked 

usually  present. 

or    sessile.     Absent   in 

or  sessile. 

adult     Cirripedia     and 

some  Copepoda. 

9.  Thorax      distinct  ; 

Thorax    with    variable 

Thorax   with  eight  seg- 

number   of    segments 

number  of  segments. 

ments,  some  of  which 

variable,  all  free. 

are    generally    united 

with  the  head. 

10.    Abdomen     distinct  ; 

Abdomen    with   variable 

Abdomen  of  seven   gen- 

variable    number      of 

number  of  separate 

erally  free    segments  ; 

fused   segments. 
11.   All  segments  of  cra- 

segments. 
Some   segments   without 

eight  in  Leptostraca. 
All  segments  usually  carry 

nidium,  thorax,  and  ab- 

appendages. 

appendages  except  the 

domen,  except  the  anal 

last  one  or  two. 

segment,  carry   paired 

appendages. 

12.   All  appendages  bira- 
mous    except     anten- 

Some     appendages     are 
modified  and  have  lost 

Some    appendages    have 
lost  biramous  structure. 

nules. 

biramous  structure. 

13.  Appendages  typically 
phyllopodiform.   Exop- 

Appendages  generally 
greatly  changed  in  most 

Appendages     typically 
phyllopodiform,       but 

odite  a  swimming  leg; 
endopodite     modified 

orders  ;  phyllopodiform 
in    young    forms    and 

greatly  modified  in  all 
but    the    lowest    order 

into  a  crawling  leg. 

throughout  life  in  Phyl- 

(Nebalia). 

lopoda. 

14.   All  appendages  of  the 
head  except  antennules 

Some  appendages  of  the 
head  modified  into  row- 

Some appendages  of  the 
head  modified  into  man- 

pediform. 

ing  organs,  mandibles, 

dibles,  or    organs    for 

or  suckers. 

seizing  food. 

15.   Thoracic  appendages 

Thoracic  appendages  am- 

Thoracic appendages  am- 

ambulatory and  swim- 

bulatory,     swimming, 

bulatory,      swimming, 

ming. 

and  seizing. 

and  seizing. 

116 


STUDIES  IN  EVOLUTION 


Sub-class  I.    Trilobita. 

Sub-class  II.     Entomostraca. 

Sub-class  III.    Malacostraca. 

16.  Abdominal  limbs  on 
all  segments  except  the 
anal,  phyllopodiform. 

17.   Coxal  elements  of  all 
limbs  forming  guatho- 
bases,    which     become 
manducatory  organs  on 
the  head. 
18.    Respiration  cuticular 
and  by  fringes  on  exop- 
odites. 

Abdominal  limbs  gener- 
ally wanting. 

Coxal    elements    seldom 
forming      gnatho  bases 
except  on  the  head. 

Respiration  mainly  cuticu- 
lar and  by  the  limbs  and 
gill  appendages. 

Abdominal  limbs  often  re- 
duced except  the  last 
pair,  which  with  telson 
frequently  form  a  caudal 
fin.     Chiefly   branchial 
in  some  groups. 
Coxal    elements    seldom 
forming      guathobases 
except    on    the    head  ; 
never  on  the  abdomen. 

Respiration  cuticular  and 
by  the  limbs  and  epipo- 
dites. 

The  more  primitive  characters  of  the  trilobites  as  drawn 
from  the  foregoing  table  may  be  summarized  as  follows: 
(1)  They  are  all  free  marine  animals ;  (2)  the  animal  has  a 
definite  configuration;  (3)  the  larva  is  a  proton auplius -like 
form ;  (4)  the  body  and  abdomen  are  richly  segmented,  and 
the  number  of  segments  is  variable ;  (5)  the  head  corresponds 
to  the  typical  crustacean;  (6)  the  thorax  and  abdomen  are 
always  distinct,  the  number  of  segments  in  each  being  vari- 
able; (7)  all  segments  except  the  anal  bear  paired  append- 
ages; (8)  all  appendages  are  typically  phyllopodiform;  and 
(9)  the  coxal  elements  of  all  limbs  form  gnathobases,  which 
become  organs  of  manducation  on  the  head. 

It  may  be  questioned  by  some  whether  the  present  state 
of  knowledge  of  the  ventral  structure  of  trilobites  warrants 
such  general  assertions  as  to  details  of  organization.  In  the 
first  place,  it  must  be  granted  that  there  is  a  remarkable 
uniformity  in  the  features  of  the  dorsal  crust,  which  natu- 
rally reflects  to  a  degree  the  differentiation  and  variation  of 
the  organs  and  appendages  of  the  ventral  side.  Further- 
more, the  actual  appendages  have  been  observed  in  such 
diverse  and  characteristic  genera  as  Trinudeus,  Triarthrus, 
Asaphus,  Ceraurus,  and  Calymmene,  and  found  to  conform 
closely  to  a  single  type,  so  that  it  seems  safe  to  assume  a  like 
agreement  throughout. 


NATURAL   CLASSIFICATION   OF  THE   TRILOBITES    117 

Morphology  of  the  Cephalon. 

The  structure  of  the  trilobite  head  suggests  homologies 
which  should  be  noticed  here,  and  if  these  correlations  are 
based  upon  true  structural  likenesses,  they  serve  not  only  to 
emphasize  the  primitive  character  of  the  trilobite,  but  also 
aid  in  interpreting  certain  organs  and  structures  in  the  higher 
Crustacea. 

The  five  fused  somites  of  the  crustacean  head  are  generally 
believed  to  correspond  to  the  third,  fourth,  fifth,  sixth,  and 
seventh  neuromeres,  leaving  the  first  and  second  unrepre- 
sented either  by  distinct  segments  or  appendages.  These 
two  neuromeres  commonly  constitute  most  of  the  cerebral 
mass  above  the  oesophagus,  and  innervate  the  ocelli  and  paired 
eyes.  In  some  the  antennae  are  innervated  from  supra- 
cesophageal  ganglia,  while  in  other  forms  their  ganglia  are 
infra-cesophageal.  It  was  formerly  supposed  that  the  stalked 
eyes  of  the  higher  Crustacea  represented  appendages  of  a 
head  segment,  but  this  belief  has  been  abandoned  on  account 
of  the  derivation  of  stalked  out  of  sessile  organs,  as  in 
Peneus,  and  also  because  the  eyes  do  not  always  have  a 
relatively  fixed  position,  but  may  pertain  to  the  first,  second, 
or  third  head  segments.  Their  structural  position  in  the 
trilobites,  however,  is  invariable,  and  it  seems  probable  that 
in  some  families  of  higher  Crustacea  the  eyes  are  in  exact 
correlation,  and  may  be  similarly  interpreted. 

The  writer5  has  previously  discussed  this  question,  and 
adduced  reasons  for  considering  the  free-cheeks  in  trilobites 
as  "the  pleura  of  an  oculiferous  head  segment."  In  many 
species  (Dalmanites,  j?Eglina,  etc.)  the  free-cheeks  are  con- 
tinuous, forming  one  piece  extending  around  the  front  of  the 
head,  between  the  cranidium  and  the  hypostoma,  while  in 
others  there  is  a  separate  piece,  the  rostral  plate,  between  the 
proximal  ends  of  the  cheek  pieces  holding  a  like  position. 
These  structures  occupy  the  exact  position  of  a  true  segment, 
and  since,  upon  theoretical  grounds,  additional  head  seg- 
ments are  to  be  accounted  for,  the  only  satisfactory  correlation 


118  STUDIES  IN  EVOLUTION 

is  to  consider  them  as  such.  Furthermore,  the  free-cheeks 
are  distinctly  separated  from  the  cranidium  by  an  open  suture, 
and  may  be  wholly  converted  into  eyes,  as  in  ^Eglina  armata 
Barrande,  or  the  unfaceted  portion  may  be  reduced  to  almost 
nothing,  as  in  Deiphon.  In  such  cases  the  parallelism  is 
exact  with  true  movable  eyes.  Bernard7  concludes  from 
his  studies  of  Apus  that  the  hypostoma  is  homologous  with 
the  annelid  prostomium.  This  would  make  the  hypostoma 
represent  the  first,  and  the  free -cheeks  the  second  of  the 
obsolete  segments.  Thus  the  trilobite  cephalon  would  fulfil 
the  demand  for  additional  evidences  of  primitive  head  seg- 
ments, and  account  for  the  development  of  eyes  separate  from 
the  cephalothorax  as  commonly  restricted. 

Supposed  evidences  of  free-cheeks  or  of  facial  sutures  have 
been  recognized  in  Limulus,  Hemiaspis,  and  Bunodes,  but 
these  seem  really  to  correspond  to  the  lines  on  the  dorsal 
surface  of  the  cephalon  of  Harpes  and  some  Trinucleu*,  run- 
ning from  the  glabella  to  the  eye-spots  and  to  the  margin, 
and  are  not  the  sutures  marking  the  limits  of  the  free  cephalic 
elements,  as  in  Asaphus  and  Proetus.  Limulus,  however, 
has  a  suture  comparable  to  that  in  Harpes  and  Trinucleus, 
extending  around  the  ventral  border  of  the  cephalothorax 
nearly  to  the  posterior  angles,  and  partly  separating  the 
ventral  plate.  In  the  process  of  moulting,  this  suture  opens 
and  enables  the  animal  to  free  itself  from  its  former  test. 

These  interpretations  may  be  employed  to  some  advantage 
in  correlating  the  segmentation  of  the  trilobite  cephalon. 
As  previously  stated,  the  recognized  plan  in  the  nervous 
system  of  a  generalized  crustacean  requires  that  there  should 
be  a  brain  or  supra-oesophageal  ganglion  innervating  (a)  the 
unpaired  eye,  (5)  the  frontal  sensory  organs  and  stalked 
eyes,  and  (c)  the  anterior  antennae;  then  a  ventral  nervous 
cord  consisting  of  a  succession  of  double  ganglia  innervating, 
respectively,  the  second  pair  of  antennae,  the  mandibles,  the 
first  pair  of  maxillae,  the  second  pair  of  maxillae,  and  lastly 
each  of  the  paired  thoracic  and  abdominal  appendages. 
Altogether,  there  are  seven  neuromeres  pertaining  to  the 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES    119 

head,  and  on  the  basis  that  each  neuromere  corresponds  to 
an  original  segment,  as  on  the  post-cephalic  region,  there 
would  need  to  be  this  number  accounted  for.  The  anterior 
segment,  or  number  one  in  the  trilobites,  would  be  repre- 
sented by  the  hypostoma ;  the  second  segment,  by  the  paired 
eyes,  free-cheeks,  and  rostral  plate;  the  third,  by  the  anterior 
lobe  of  the  glabella  and  the  first  antennae ;  the  fourth,  by  the 
second  lobe  of  the  glabella  and  the  second  pair  of  antennae ; 
the  fifth,  by  the  third  lobe  of  the  glabella  and  the  mandibles ; 
the  sixth,  by  the  fourth  lobe  of  the  glabella  and  the  first 
maxillee;  and  the  seventh,  by  the  neck  lobe,  or  occipital 
ring,  and  the  second  pair  of  maxillae.  The  five  annulations, 
or  lobes,  of  the  axis  of  the  cranidium,  since  they  primarily 
carry  fulcra  for  the  attachment  of  muscles  supporting  or 
moving  the  appendages,  could  thus  be  interpreted  in  terms  of 
the  ventral  structure,  making  the  first  lobe  the  antennulary, 
the  second  the  antennary,  the  third  the  mandibular,  the 
fourth  the  first  maxillary,  and  the  fifth  the  second  maxillary. 
No  other  group  of  Crustacea*  furnishes  such  constant  and 
well-developed  structures  representing  the  second  theoretical 
head  segment,  which  is  obscure  or  obsolete  in  all  the  living 
groups,  excepting  probably  the  stalked  eyes  of  some  Crus- 
tacea and  the  movable  ocular  segment  of  the  Stomatopoda. 
For  this  reason,  in  addition  to  the  many  other  important 
differences  previously  noted  in  the  table  of  comparative 
morphology,  the  trilobites  are  regarded  as  a  sub-class,  and 
the  relative  denomination  and  structural  relations  of  this 
second  segment,  along  with  other  characters,  are  considered 
as  of  sufficient  physiological  and  morphological  importance  to 
determine  the  ordinal  divisions. 

Principles  of  a  Natural  Classification. 

Most  satisfactory  and  conclusive  results  have  already  come 
from  the  application  of  the  law  of  morphogenesis,  or  the 
recapitulation  theory,  to  various  groups  of  animals,  by  means 
of  which  their  natural  classification  and  phylogenetic  rela- 
tions have  been  determined.  Hyatt21  says  on  this  point 


120  STUDIES  IN  EVOLUTION 

(1889):  "We  have  endeavored  to  demonstrate  that  a  natural 
classification  may  be  made  by  means  of  a  system  of  analysis 
in  which  the  individual  is  the  unit  of  comparison,  because 
its  life  in  all  its  phases,  morphological  and  physiological, 
healthy  or  pathological,  embryo,  larva,  adolescent,  and  old 
(ontogeny),  correlates  with  the  morphological  and  physiolog- 
ical history  of  the  group  to  which  it  belongs  (phylogeny). " 
It  is  also  interesting  to  note  that  Agassiz 1  recognized  in 
ontogeny  a  standard  of  classification.  One  of  his  strongest 
statements  is  as  follows :  "  Embryology  [=  ontogeny]  will  in 
the  end  furnish  us  with  the  means  of  recognizing  the  true 
affinities  among  all  animals,  and  of  ascertaining  their  relative 
standing  and  normal  position  in  their  respective  classes  with 
the  utmost  degree  of  accuracy  and  precision." 

These  principles  can  be  best  applied  in  a  group  of  animals 
which  has  a  geological  history  more  or  less  complete,  and 
which  is  not  wholly  parasitic  or  greatly  degenerated.  It  is 
of  the  greatest  importance,  also,  to  study  the  ontogeny  of 
primitive  and  non-specialized  species,  because  without  very 
complete  paleontological  evidence  the  development  of  a  much 
later  derived  form  may  be  so  involved  with  larval  adaptations 
and  accelerated  characters  as  to  be  misleading. 

The  trilobites  lend  themselves  to  this  treatment  in  fulfil- 
ling most  of  the  necessary  conditions.  They  have  a  known 
geological  history  stretching  through  the  entire  Paleozoic, 
from  the  beginning  of  the  Cambrian  to  the  Permian.  Their 
structure  is  generalized  and  quite  uniform,  and  no  sessile, 
attached,  parasitic,  land,  or  freshwater  species  are  known. 
The  ontogeny  of  all  the  principal  groups  has  been  studied, 
including  Cambrian,  Ordovician,  Silurian,  and  Devonian 
types. 

The  trilobites  necessarily  furnish  little  information  of  the 
stages  of  growth  which  may  be  classed  as  embryonic.  The 
early  embryonic  stages  are  not  preserved  as  fossils,  and  there- 
fore may  be  omitted.  In  this  category  are  the  protembryo, 
or  the  ovum  in  its  unsegmented  and  segmented  stages  (the 
so-called  "eggs  of  trilobites"  may  of  course  represent  any 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES    121 

stage  of  embryonic  development  before  the  escape  of  the 
young);  the  mesembryo,  or  bias  tosph  ere ;  the  metembryo,  or 
gastrula;  the  neoembryo,  or  planula-like  stage;  and  the 
typembryo,  when  the  first  distinctive  features  make  their 
appearance.  The  first  embryonic  stage  recognized  in  the 
trilobites  can  be  referred  to  the  phylembryo,  as  defined  by 
Jackson,22  when  the  animal  may  be  clearly  referred  to  its 
proper  class.  Since  this  period  is  distinctive  for  each  class 
of  animals  and  usually  bears  a  separate  name,  it  has  been 
termed  by  the  writer5  the  protaspis  stage  of  trilobites.  It 
closely  approximates  the  protonauplius  form,  or  the  theoret- 
ical, primitive,  ancestral  larval  form  of  the  Crustacea.  Like 
the  homologous  nauplius  of  modern  higher  Crustacea,  it  is 
the  characteristic  larval  type  common  to  the  class.  The 
nauplius  is  therefore  considered  as  a  derived  larva  modified 
by  adaptation. 

The  post-embryonic  stages  of  ontogeny  have  received  the 
names  nepionic,  for  the  infantile  or  young;  neanic,  for  the 
immature  or  adolescent;  ephebic^  for  the  mature  or  adult; 
and  gerontic,  for  the  senile  or  old.  When  especially  applied 
to  trilobites,  the  nepionic  stages  may  include  the  animal 
when  the  cephalon  and  pygidium  are  distinct  and  the  thorax 
incomplete.  There  would  thus  be  as  many  nepionic  stages 
as  the  number  of  thoracic  segments.  The  neanic  stages 
would  be  represented  by  the  animal  with  all  parts  complete, 
but  with  the  average  growth  incomplete.  Final  progressive 
growth  and  development  of  the  individual  would  fall  under 
the  ephebic  stage.  Lastly,  general  evidences  of  senility 
would  be  interpreted  as  belonging  to  the  gerontic  stage. 

Application  of  Principles  for  Ordinal  Divisions. 

In  other  classes  of  animals  above  the  lower  coelenterates, 
the  ph}'lembryonic  stage  is  the  starting-point  from  which 
correlations  are  made,  and  out  of  which  all  the  higher  groups 
are  developed  by  a  series  of  changes  along  certain  lines. 
The  protoconch  represents  this  period  in  the  cephalopods 


122  STUDIES  IN  EVOLUTION 

and  gastropods;  the  prodissoconch,  in  the  pelecypods;  the 
protegulum,  in  the  brachiopods,  and  the  protechinus,  in  the 
echinoids.  In  the  trilobites  the  protaspis,  as  already  stated, 
has  the  value  of  the  phylembryo,  and  in  its  geological  history 
and  the  metamorphoses  it  undergoes  to  produce  the  perfect 
trilobite  accurate  information  can  be  gained  as  to  what  the 
primitive  characters  are,  and  the  relative  values  of  other 
features  acquired  during  the  long  existence  of  the  class. 

The  simple  characters  possessed  by  the  protaspis  are  the 
following,  as  drawn  from  the  study  of  this  stage  in  all  the 
principal  groups  of  trilobites :  Dorsal  shield  minute,  not  more 
than  .4  to  1  mm.  in  length;  circular  or  ovate  in  form;  axis 
distinct,  more  or  less  strongly  annulated,  limited  by  longi- 
tudinal grooves;  head  portion  predominating;  axis  of  cranid- 
ium  with  five  annulations;  abdominal  portion  usually  less 
than  one-third  the  length  of  the  shield ;  axis  with  from  one 
to  several  annulations;  pleural  portion  smooth  or  grooved; 
eyes,  when  present,  anterior,  marginal,  or  sub-marginal ;  free- 
cheeks,  when  visible,  narrow  and  marginal.  Examples, 
Plate  II,  figures  1,  5. 

During  this  stage  several  moults  took  place  before  the 
complete  separation  of  the  pygidium  or  the  introduction  of 
thoracic  segments.  These  brought  about  various  changes ;  as 
the  stronger  annulation  of  the  axis,  the  appearance  of  the 
free-cheeks  on  the  dorsal  side,  and  the  growth  of  the  pygid- 
ium by  the  introduction  of  new  appendages  and  segments, 
as  indicated  by  the  additional  grooves  on  the  axis  and  limb. 
A  full  representation  of  the  variety  and  succession  of  these 
early  protaspis  stages  is  presented  in  the  writer's  paper  on 
the  "Larval  Stages  of  Trilobites."6  Some  of  the  conclusions 
and  discussions  in  that  paper  are  made  use  of  here. 

In  the  earliest  or  Cambrian  genera  the  protaspis  stage  is 
by  far  the  simplest  expression  of  this  period  to  be  found.  In 
the  higher  and  later  genera  the  process  of  acceleration  or 
earlier  inheritance  has  pushed  forward  certain  characters 
until  they  appear  in  the  protaspis,  thus  making  it  more  and 
more  complex. 


NATURAL   CLASSIFICATION  OF  THE   TPJLOBITES    123 

Taking  the  early  protaspis  stages  in  Solenopleura,  Lios- 
tracus,  or  Pty  chop  aria,  it  is  found  that  they  agree  exactly 
with  the  foregoing  diagnosis  in  its  most  elementary  sense. 
Since  they  are  the  characters  shared  in  common  by  all  the 
larvae  at  this  stage,  they  are  taken  as  primitive  and  accorded 
that  value  in  dealing  with  adult  forms  possessing  homolo- 
gous features.  Therefore  any  trilobite  with  a  large  elongate 
cephalon,  eyes  rudimentary  or  absent,  free-cheeks  ventral 
or  marginal,  and  glabella  long,  cylindrical,  and  with  five 
annulations,  would  naturally  be  placed  near  the  beginning  of 
any  genetic  series  or  as  belonging  to  a  very  primitive  stock. 

Next  must  be  considered  the  progressive  addition  of  char- 
acters during  the  geological  history  of  the  protaspis,  and  in 
the  ontogeny  of  the  individual  during  its  growth  from  the 
larval  to  the  mature  condition.  It  was  shown  in  the  paper 
already  referred  to,  that  there  was  an  exact  correlation  to  be 
made  between  the  geological  and  zoological  succession  of 
first  larval  stages  and  adult  forms,  and  therefore  both  may 
be  reviewed  together. 

The  first  important  structures  not  especially  noticeable  in 
all  stages  of  the  protaspis  are  the  free-cheeks,  which  usually 
manifest  themselves  in  the  meta-  or  para-protaspis  stages, 
though  sometimes  even  later.  Since  they  bear  the  visual 
areas  of  the  eyes,  when  they  are  present,  their  appearance 
on  the  dorsal  shield  is  practically  simultaneous  with  these 
organs,  and  before  the  eyes  have  travelled  over  the  margin 
the  free-cheeks  must  be  wholly  ventral  in  position.  When 
first  discernible  they  are  very  narrow,  and  in  Ptyclioparia  and 
Sao  include  the  genal  angles.  In  Dalmanites  and  Cheirurus, 
however,  the  genal  angles  are  borne  on  the  fixed-cheeks.  If, 
as  Bernard 7  concludes,  the  crustacean  head  has  been  formed 
by  the  bending  under,  to  the  ventral  side,  of  the  anterior 
segments  of  an  ancestral  carnivorous  annelid,  this  furnishes 
a  means  of  further  determining  and  also  of  satisfactorily 
correlating  the  prime  significance  and  importance  of  the  free- 
cheeks. 

Since   the   free-cheeks   are  ventral   in   the  earliest  larval 


124  STUDIES  IN  EVOLUTION 

stages  of  all  but  the  highest  trilobites,  and  as  this  is  an  adult 
feature  among  a  number  of  genera  which  on  other  grounds 
are  very  primitive,  this  is  taken  as  generally  indicative  of  a 
very  low  rank.  It  seems  to  mean  that  the  second  segment 
remains  where  it  was  mechanically  placed,  and  retains  its 
full  somitic  nature,  though  from  the  necessities  of  such  a 
condition  true  ventral  segments  must  soon  disappear  through 
modification  into  other  structures  or  through  disuse  as  seg- 
ments. The  genera  Harpes,  Agnostus,  Trinudeus,  and  their 
allies  agree  in  having  well-developed,  continuous,  ventral 
free-cheeks,  and  constitute  a  natural  group.  As  they  pos- 
sess one  expression  or  type  of  the  genesis  of  an  important 
common  character,  based  upon  facts  of  development,  it  should 
stand  as  an  ordinal  character,  and  as  such  it  is  here  taken. 
For  this  group  the  name  HYPOPARIA  is  proposed.  It  is  fully 
defined  and  its  limitations  established  in  the  proper  place  in 
the  classification. 

The  remaining  genera  of  trilobites  present  two  distinct 
types  of  head  structure,  dependent  upon  the  extent  and  char- 
acter of  the  free-cheeks.  In  both,  the  free -cheeks  make  up 
an  essential  part  of  the  dorsal  crust  of  the  cephalon,  being 
continued  on  the  ventral  side  only  as  a  doublure  or  infolding 
of  the  edge,  similar  to  that  of  the  free  edge  of  the  cranidium, 
the  ends  of  the  thoracic  pleura,  and  the  margin  of  the 
pygidium.  They  may  be  separated  only  by  the  cranidium, 
as  in  Ptychoparia,  or  by  the  cranidium  and  rostral  plate,  as  in 
Illcenus  and  Homalonotus,  or  they  may  be  united  and  con- 
tinuous in  front,  as  in  JEglina  and  Dalmanites.  One  type  of 
structure  is  distinguished  by  having  the  free-cheeks  include 
the  genal  angles,  thus  cutting  off  more  or  less  of  the  pleura 
of  the  occipital  segment.  The  genera  belonging  to  this 
group  constitute  the  second  order,  the  OPISTHOPARIA. 

The  third  and  last  type  of  structure  includes  forms  in 
which  the  pleura  of  the  occipital  segment  extend  the  full 
width  of  the  base  of  the  cephalon,  embracing  the  genal 
angles.  The  free-cheeks  are  therefore  separated  from  the 
cranidium  by  sutures  cutting  the  lateral  margins  of  the 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES    125 

cephalon  in  front  of  the  genal  angles.     Genera  having  this 
structure  are  here  placed  in  the  order  PROPAKIA. 

Several  genera,  as  Calymmene  and  Triarthrus^  have  been 
described  as  having  the  facial  sutures  beginning  at  or  cutting 
the  apex  of  the  genal  angle,  thus  making  it  indeterminate 
whether  they  should  be  classed  with  the  Opisthoparia  or 
Proparia.  It  will  be  found,  however,  that  some  species  of 
these  genera  leave  no  doubt  as  to  the  anterior  or  posterior 
position  of  the  suture.  The  small  genal  spines  of  Calymmene 
callicephala  Green  are  situated  on  the  ends  of  the  fixed- 
cheeks,  while  similar  but  larger  spines  in  Triarthrus  spinosus 
Billings  are  on  the  free-cheeks,  making  the  former  belong  to 
the  Proparia  and  the  latter  to  the  Opisthoparia. 


Application  of  Principles  for  Arrangement  of  Families 
and  G-enera. 

The  remaining  characters  to  be  noticed  have  chiefly  family 
and  generic  values,  and  naturally ,  follow  the  preceding  dis- 
cussions. They  are  of  great  assistance  both  in  determining 
the  place  of  a  family  in  an  order,  and  the  rank  and  genetic 
position  of  a  genus  in  a  family. 

There  is  very  satisfactory  evidence  that  the  eyes  have 
migrated  from  the  ventral  side,  first  forward  to  the  margin 
and  then  backward  over  the  cephalon  to  their  adult  position. 
The  most  primitive  Iarva3  should  therefore  present  no  evi- 
dence of  eyes  on  the  dorsal  shield.  Just  such  conditions  are 
fulfilled  in  the  youngest  larva  of  Ptychoparia,  Solenopleura, 
and  Liostracus.  The  eye -line  is  present  in  the  later  larval 
and  adolescent  stages  of  these  genera,  and  persists  to  the 
adult  condition.  In  Sao  it  has  been  pushed  forward  to  the 
earliest  protaspis,  and  is  also  found  in  the  two  known  larval 
stages  of  Triarthrus.  Sao  retains  the  eye-line  throughout 
life,  but  in  Triarthrus  the  adult  has  no  trace  of  it.  A 
study  of  the  genera  of  trilobites  shows  that  this  is  a  very 
archaic  feature,  chiefly  characteristic  of  Cambrian  genera, 
and  only  appearing  in  the  primitive  genera  of  higher  and 


126  STUDIES  IN  EVOLUTION 

later  groups  or  as  an  evidence  of  degeneration.  It  first 
develops  in  the  later  larval  stages  of  certain  genera  (PtycJio- 
paria,  etc.);  next  in  the  early  larval  stages  (Sao)-,  then  dis- 
appears from  the  adult  stages  (Triarthrus) ;  and  finally  is 
pushed  out  of  the  ontogeny  (Dalmanites). 

In  Ptychoparia,  /Solenopleura,  Liostracus,  Sao,  and  Triar- 
thrus  the  eyes  are  first  visible  on  the  margin  of  the  dorsal 
shield  after  the  protaspis  stages  have  been  passed  through, 
and  later  than  the  appearance  of  the  eye-lines ;  but  in  Proetus, 
Acidaspis,  Arges,  and  Dalmanites,  through  acceleration,  they 
are  present  in  all  the  protaspis  stages,  and  persist  to  the 
mature  or  ephebic  condition,  moving  in  from  the  margin  to 
near  the  sides  of  the  glabella.  Progression  in  these  char- 
acters may  be  expressed,  and  in  so  far  taken  for  general 
application  among  adult  forms  to  indicate  rank,  as  follows : 
(1)  Absence  of  eyes;  (2)  eye-lines;  (3) eye-lines  and  marginal 
eyes;  (4)  marginal  eyes;  (5)  sub-marginal  eyes;  (6)  eyes  near 
the  pleura  of  the  neck  segment. 

The  changes  in  the  glabella  are  equally  important  and 
interesting.  Throughout  the  larval  stages  the  axis  of  the 
cranidium  shows  distinctly  by  the  annulations  that  it  is 
composed  of  five  fused  segments,  indicating  the  presence  of 
as  many  paired  appendages  on  the  ventral  side.  In  its  sim- 
plest and  most  primitive  state  it  expands  in  front,  joining 
and  forming  the  anterior  margin  of  the  head  (larval  Ptycho- 
paria and  Sao).  During  later  growth  it  becomes  rounded  in 
front  and  terminates  within  the  margin.  In  higher  genera, 
through  acceleration,  it  is  rounded  and  well  defined  in  front, 
even  in  the  earliest  larval  stages,  and  often  ends  within  the 
margin  (larval  Triartlirm  and  Acidaspis).  From  these  simple 
types  of  simple  pentamerous  glabellsB  all  the  diverse  forms 
among  species  of  various  genera  have  been  derived,  through 
changes  affecting  any  or  all  the  lobes.  The  modifications 
usually  consist  in  the  progressive  obsolescence  of  the  ante- 
rior annulations,  finally  producing  a  smooth  glabella,  as  in 
Ulcenus  and  Niobe.  The  neck  segment  is  the  most  persistent 
of  all,  and  is  rarely  obscured.  The  third,  or  mandibular, 


NATURAL    CLASSIFICATION  OF  THE   TRILOBITES    127 

segment  is  frequently  marked  by  two  entirely  separate  lateral 
lobes,  as  in  Acidaspis,  Oonolichas,  Chasmops,  etc.  Likewise, 
the  fourth  annulation  carrying  the  first  pair  of  maxillae  is 
often  similarly  modified  in  the  same  genera,  also  in  all  the 
Proetidse,  and  in  Cheirurus,  Crotalocephalus,  Sphoerexochus, 
Ampyx,  Harpes,  etc.  Here  again,  among  adult  forms,  the 
stages  of  progressive  differentiation  may  be  taken  as  indi- 
cating the  relative  rank  of  the  genera. 

The  comparative  areal  growth  of  the  free-cheeks  is  ex- 
pressed by  the  gradual  moving  of  the  facial  suture  toward 
the  axis.  As  the  free-cheeks  become  larger  the  fixed-cheeks 
become  smaller.  In  the  most  primitive  protaspis  stages  and 
in  Agnostics,  Harpes,  and  Trinucleus  the  dorsal  surface  of  the 
cephalon  is  wholly  occupied  by  the  axis  and  the  fixed-cheeks, 
while  in  the  higher  genera  the  area  of  the  fixed-cheeks 
becomes  reduced  until,  as  in  Stygina  and  Phillipsia,  they 
form  a  mere  border  to  the  glabella.  Therefore  the  ratio 
between  the  fixed-  and  free-cheeks  furnishes  another  means 
of  assisting  in  the  determination^  of  rank. 

The  pleura  from  the  segments  of  the  glabella  are  occasion- 
ally visible,  as  in  the  young  of  Elliptocephala,  but  usually 
the  pleura  of  the  neck  segments  are  the  first  and  only  ones  to 
be  distinguished  on  the  cephalon,  the  others  being  so  com- 
pletely coalesced  as  to  lose  all  traces  of  their  individuality. 
The  pleura  of  the  pygidium  appear  soon  after  the  earliest 
protaspis  stage,  and  in  some  genera  (Sao,  Dalmanites)  are 
even  more  strongly  marked  than  in  the  adult  state  and  much 
resemble  separate  segments.  The  growth  of  the  pygidium  is 
very  considerable  through  the  protaspis  stage.  At  first  it 
is  less  than  one-third  the  length  of  the  dorsal  shield,  but  by 
successive  addition  of  segments  it  soon  becomes  nearly  one- 
half  as  long.  In  some  genera  it  is  completed  before  the 
appearance  of  the  free  thoracic  segments,  all  of  which  are 
added  during  the  nepionic  stages.  An  interpretation  of 
these  facts  to  apply  in  valuing  adult  characters  would  indi- 
cate that  a  very  few  segments,  both  in  thorax  and  pygidium, 
may  be  evidence  of  arrested  development  or  degeneration. 


128  STUDIES  IN  EVOLUTION 

On  the  other  hand,  the  apparently  unlimited  multiplication 
of  thoracic  and  especially  of  abdominal  segments  in  some 
genera  is  also  to  be  considered  as  a  primitive  character 
expressive  of  an  annelidan  style  of  growth.  Genera  like 
Asaphus,  Phacops,  etc.,  having  a  constant  number  of  thoracic 
segments  accompanied  by  other  characters  of  a  high  order, 
undoubtedly  represent  the  normal  trilobite  type. 

These  analyses  and  correlations  clearly  show  that  there  are 
characters  appearing  in  the  adults  of  later  and  higher  genera, 
which  successively  make  their  appearance  in  the  protaspis 
stage,  sometimes  to  the  exclusion  or  modification  of  struc- 
tures present  in  the  most  primitive  larvae.  Thus  the  larvae 
of  Dalmanites  or  Proetus,  with  their  prominent  eyes  and 
glabella  distinctly  terminated  and  rounded  in  front,  have 
characters  which  do  not  appear  in  the  larval  stages  of  ancient 
genera,  but  which  may  come  in  their  adult  stages.  Evi- 
dently such  modifications  have  been  acquired  by  the  action 
of  the  law  of  earlier  inheritance,  or  tachygenesis. 

In  a  classification  of  trilobites,  for  the  purpose  of  illustrat- 
ing the  principles  here  enunciated,  the  ontogenies  of  Sao  and 
Dalmanites,  Plate  II,  figures  1-8,  are  selected.  Sao  belongs 
to  the  ancient  family  Olenidse  of  the  order  Opisthoparia,  and 
naturally  may  be  expected  to  furnish  very  clear  evidence  as 
to  the  relations  of  many  lower  and  older  genera.  Dalmanites, 
also,  with  its  simple  head  structure,  will  give  similar  data 
regarding  the  Proparia. 

The  early  protaspis  stage  of  Sao,  Plate  II,  figure  1,  has 
no  dorsal  development  of  the  free-cheeks,  and  with  the  elon- 
gate form  of  the  cephalic  portion  may  be  compared  with  the 
cephala  of  Agnostus  and  Microdiscus,  and  therefore  correlates 
with  the  Hypoparia.  The  cephalon,  at  a  later  period  of 
development,  when  the  animal  has  two  free  thoracic  seg- 
ments, Plate  II,  figure  2,  shows  the  narrow  marginal  free- 
cheeks  and  distinct  eye-lines.  Here  the  resemblance  to  the 
cephala  of  Atops  and  Conocoryptie,  Plate  II,  figures  14,  15, 
is  very  marked,  and  indicates  that  the  Conocoryphidas  are 
genetically  the  first  family  of  the  Opisthoparia.  When  Sao 


NATURAL    CLASSIFICATION  OF    THE   TRILOBITES    129 

has  eight  thoracic  segments,  Plate  II,  figure  3,  the  characters 
of  the  cephalon  accord  closely  with  Ptychoparia  and  Olenus. 
showing  that  these  genera  should  precede  it  in  arranging  the 
genera  of  the  family  Olenidse.  Evidence  is  thus  furnished 
for  the  proper  position  of  the  first  two  families  of  the  order. 
Now,  if  the  relative  values  of  the  differentiation  of  the 
glabella,  the  position  of  the  eyes,  and  the  size  of  the  free- 
cheeks  are  considered  in  the  light  of  the  preceding  analyses 
of  these  features,  the  remaining  families  of  the  order,  as 
represented  in  their  typical  genera,  naturally  arrange  them- 
selves as  indicated  in  Plate  II,  figures  18-23.  There  result 
(1)  the  Conocoryphidse  (represented  by  Atops  and  Cono- 
coryphe,  figures  14,  15) ;  (2)  the  Olenidse  (Ptychoparia  and 
Olenus,  figures  16,  17);  (3)  the  Asaphidse  (Asaphus  and 
lllcenus,  figures  18,  19);  (4)  the  Proetidse  (Proetus,  figure 
20);  (5)  the  Bronteidse  (Bronteus,  figure  21);  (6)  the  Lichad- 
idas  (Lichas,  figure  22);  and  (7)  the  Acidaspidse  (Acidaspis, 
figure  23). 

For  the  Proparia  similar  results  are  brought  out  by  the 
study  of  the  ontogeny  of  Dalmanites  and  by  comparisons 
with  the  characters  governing  the  sequence  of  families  in  the 
Opisthoparia.  The  narrow  marginal  free-cheeks  place  the 
Encrinuridas  and  Calymmenidae  as  primitive.  The  small  or 
obsolete  eyes  and  the  larval  form  of  the  glabella  in  the  former 
further  show  that  this  family  should  be  placed  at  the  begin- 
ning. The  nepionic  Dalmanites,  with  seven  thoracic  segments, 
has  a  head  structure  very  similar  to  the  adult  Oheirurus 
(Eccoptocheile),  figure  28,  thus  making  the  Cheiruridae  pre- 
cede the  Phacopidae.  The  arrangement  of  families  under  the 
Proparia  accordingly  will  be  (1)  the  Encrinuridae  (Placoparia 
and  Encrinurus,  Plate  II,  figures  24,  25) ;  (2)  the  Calym- 
menidse  (Calymmene  and  Dipleura,  figures  26,  27);  (3)  the 
Cheiruridae  (Oheirurus  (Eccoptocheile) ,  figure  28);  and  (4) 
the  Phacopidae  {Dalmanites,  Chasmops,  Acaste,  Phacops, 
figures  29-33). 

The  sequence  of  families  in  the  most  primitive  order, 
Hypoparia,  may  now  be  easily  disposed  of.  The  genera  are 

9 


130  STUDIES  IN  EVOLUTION 

so  aberrant  and  offer  such  conspicuous  differences  from  ordi- 
nary trilobites  that  it  was  considered  better  to  delay  their 
disposition  until  the  variations  in  structure  governing  the 
arrangement  of  families  in  the  higher  orders  were  clearly 
shown.  The  degree  of  specialization  of  the  glabella,  of  the 
form  and  character  of  the  fixed-cheeks,  and  the  great  range 
in  the  number  of  segments  in  the  thorax  and  pygidium  are 
strong  evidence  that  we  are  dealing  with  the  terminal  genera 
of  the  order,  which  must  have  attained  its  normal  develop- 
ment in  pre-Cambrian  times.  Agnostus  and  Microdiscus 
have  so  many  protaspidian  and  larval  characters  that  they 
must  be  considered  more  primitive  than  the  other  genera, 
although  in  some  respects  they  show  a  high  degree  of  speciali- 
zation and  even  degeneration,  as  will  be  noticed  under  the 
family  Agnostidse.  Moreover,  Harpes,  in  its  elongate  cepha- 
lon,  persistent  ocelli,  and  many  thoracic  segments,  is  also 
quite  primitive.  Trinucleus,  with  ocelli  present  only  in 
larval  stages,  a  transverse  cephalon,  and  genal  spines  belong- 
ing to  the  free-cheeks,  is  considerably  higher  and  properly 
comes  last  in  the  order,  thus  making  the  arrangement  of 
families  as  follows:  (1)  Agnostidse  (Agnortus,  Microdiscus, 
Plate  II,  figures  9,  10);  (2)  Harpedidse  (Harpes,  figure  11); 
and  (3)  Trinucleidse  (Trinucleus,  Ampyx,  figures  12,  13). 

Diagnoses  and  Discussions. 
Sub-class  TRILOBITA. 

Marine  Crustacea,  with  a  variable  number  of  metameres ;  body 
covered  with  a  hard  dorsal  shell  or  crust,  longitudinally  trilo- 
bate from  the  defined  axis  and  pleura;  cephalon,  thorax,  and  abdo- 
men distinct.  Cephalon  covered  with  acephalic  shield  composed 
of  a  primitively  pentamerous  middle  piece,  the  cranidium,  and 
two  side  pieces,  or  free-cheeks,  which  may  be  separate  or  united 
in  front,  and  carry  the  compound  sessile  eyes,  when  present; 
cephalic  appendages  pediform,  consisting  of  five  pairs  of  limbs, 
all  biramous,  and  functioning  as  ambulatory  and  oral  organs, 
except  the  simple  antennules,  which  are  purely  sensory.  Upper 
lip.  forming  a  well-developed  hypostoma;  under  lip  present. 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES     131 

Somites  of  the  thorax  movable  upon  one  another,  varying  in 
number  from  two  to  twenty-nine.  Abdominal  segments  vari- 
able in  number,  and  fused  to  form  a  caudal  shield.  All  seg- 
ments, thoracic  and  abdominal,  carry  a  pair  of  jointed  biramous 
limbs.  All  limbs  have  their  coxal  elements  forming  gnath- 
obases,  which  become  organs  of  manducation  on  the  head. 
Eespiration  integumental  and  by  branchial  fringes  on  the  exop- 
odites.  Development  proceeding  from  a  protonauplius  form, 
by  the  progressive  addition  of  segments  at  successive  moults. 

Heretofore  it  has  been  impossible  to  give  an  adequate 
diagnosis  of  the  Trilobita,  owing  to  the  absence  of  informa- 
tion regarding  certain  important  characters,  and  the  obscurity 
of  the  information  relating  to  some  other  features.  It  is 
believed  that  enough  is  now  known  to  frame  a  definition  of 
the  class,  which,  in  accuracy  and  completeness,  will  compare 
favorably  with  any  based  upon  living  groups.  Such  a  defi- 
nition brings  out  the  fact  that  the  differences  between  the 
trilobites  and  other  large  groups  are  clearly  recognizable,  and 
do  not  consist  of  a  statement  of  anomalous  characters  whose 
real  significance  is  unknown. 

Arrangement  of  the  Families  of  Trilobites. 

SUB-CLASS  TRILOBITA. 

Order  A.    HYPOPARIA. 

Family  1.   Agnostidae.  Family  3.   Trinucleidae. 

Family  2.   Harpedidae. 

Order  B.    OPISTHOPARIA. 

Family  4.  Conocoryphidae.  ,  Family  8.   Bronteidae. 

Family  5.  Olenidae.  Family  9.    Lichadidae. 

Family  6.  Asaphidae.  Family  10.   Acidaspidae. 

Family  7.  Proetidae. 

Order  C.   PROPARIA. 

Family  11.   Encrinuridae.  Family  13.    Cheiruridae. 

Family  12.    Calymmenidae.        Family  14.    Phacopidae. 

The  order  Opisthoparia,  with  nearly  one  hundred  and  fifty 
genera,  has  a  much  greater  geological  distribution  than  either 


132  STUDIES  IN  EVOLUTION 

of  the  others,  and  was  by  far  the  dominant  group  during  the 
Cambrian  and  Ordovician,  being  represented  by  about  eighty- 
five  genera  in  the  former  age  and  forty-five  in  the  latter. 
Nineteen  genera  of  this  order  are  found  in  the  Silurian  and 
ten  in  the  Devonian,  most  of  them  having  continued  on  from 
older  ages.  Four  genera  represent  the  order  in  the  Car- 
boniferous and  one  in  the  Permian,  thus  marking  the  ex- 
tinction of  the  sub-class  as  well  as  the  last  genera  of  the 
Opisthoparia. 

The  comparative  abundance  and  duration  of  the  three 
orders  are  expressed  in  the  table  on  page  133,  from  which 
it  appears  that  the  Hypoparia  probably  culminated  in  pre- 
Cambrian  times,  the  Opisthoparia  during  the  Cambrian,  and 
the  Proparia  during  the  Ordovician. 

In  the  following  classification  the  families  adopted  by 
Salter 32  and  Barrande 3  are  in  the  main  adhered  to,  and  the 
number  corresponds  very  closely  with  that  in  Zittel's  "  Hand- 
buch  der  Palseontologie  "  ®>  and  also  in  the  "  Grundziige"  R6  of 
the  same  author.  The  order  of  arrangement,  however,  is 
very  different.  A  great  number  of  family  divisions  have 
been  proposed,  and  undoubtedly  many  others  will  yet  be 
made,  but  it  is  not  within  the  province  of  this  paper  to  deter- 
mine the  precise  value  and  limitations  of  the  families.  This 
would  require  discussions  of  priority  and  synonymy,  and  other- 
wise detract  from  the  direct  purpose  of  the  writer;  namely, 
to  establish  a  basis  for  a  natural  classification,  and  in  this 
way  to  apply  what  is  currently  known  and  accepted  regard- 
ing the  trilobites.  Nevertheless,  some  notice  must  be  taken 
of  several  families  and  genera  which  for  various  reasons  do 
not  appear  here.  The  family  Aglaspidae,  including  the  genus 
Aglaspis  Hall,  proves  to  belong  to  the  Merostomata  and  is 
therefore  omitted.  The  family  Bohemillidse  has  been  shown 
by  the  writer6  to  have  no  foundation,  because  the  type  of 
the  genus  Bohemilla  Barrande  was  based  upon  a  mutilated 
specimen  of  ^glina. 

Several  genera  still  commonly  adopted  are  not  here  recog- 
nized in  the  lists  under  the  families,  since  from  the  minute 


NATURAL   CLASSIFICATION  OF  THE   TRILOBITES    133 

size  of  the  individuals  described  and  their  immature  char- 
acters they  must  be  considered  as  the  young  of  larger  forms. 
Such  are  Conophrys  Callaway,  Cyphoniscus  Salter,  Holo- 
metopus  Angelin,  Isocolus  Angelin,  and  Shumardia  Billings. 
Triopus  Barrande  has  been  shown  to  be  a  chiton. 


FIGURE  75. —  Table  of  Geological  Distribution  of  Trilobita. 

Much  could  be  said  against  some  of  the  recognized  genera, 
but,  as  with  the  families,  the  writer  has  preferred  in  almost 
every  case  to  adopt,  for  the  present,  what  has  been  commonly 
accepted,  and  thus  to  avoid  the  entanglement  of  dates  and 
synonyms  which  would  be  out  of  place  in  any  general  discus- 
sions. The  type  species  of  every  genus  is  here  made  the 
central  idea.  It  is  taken  as  representing  the  genus  more 
closely  than  any  fortuitous  assemblage  of  diverse  species, 
which  the  next  investigator  may  show  belong  to  another  or 
to  several  genera.  Our  ideas  of  a  genus  are  naturally  based 
mainly  upon  the  species  with  which  we  are  most  familiar. 
Until  forced  to  make  authoritative  comparative  statements, 
it  does  not  occur  to  one  that  the  type  of  the  genus  under 
consideration  may  be  quite  different.  An  American  stu- 
dent's conception  of  Homalonotus  will  probably  be  formed 


134  STUDIES  IN  EVOLUTION 

largely  upon  the  species  commonly  known  as  H.  delphino- 
cephalus  Green,  from  the  Niagara,  and  H.  DeKayi  Green, 
from  the  Hamilton.  The  first  time  the  type  of  the  genus, 
H.  Knighti  Murchison,  is  seen  he  will  be  puzzled  to  place  it. 
Similar  examples  could  be  multiplied  indefinitely,  and  only 
show  that  the  type  must  be  taken  as  the  ultimate  unit  of 
comparison. 

Diagnoses  and  Discussions  of  Orders  and  Families. 

Order  A.  HYPOPAKIA,  nov.  ord. 

(UTTO  under,  and  irapfia  cheek  piece.) 

Free-cheeks  forming  a  continuous  marginal  ventral  plate  of 
the  cephalon,  and  in  some  forms  also  extending  over  the  dorsal 
side  at  the  genal  angles.  Suture  ventral,  marginal,  or  suhmar- 
ginal.  Compound  paired  eyes  absent;  simple  eyes  may  occur 
on  each  fixed-cheek,  singly  or  in  pairs. 

Including  the  families  Agnostidse,  Harpedidse,  and  Trinucleidse. 

This  order  includes  the  groups  C  and  D,  or  the  Ampycini 
and  Agnostini  of  Salter,  and  also  the  family  HarpedidaB  of 
that  author,  which  he  included  in  the  Asaphini.  The  special 
recognition  of  characters,  however,  between  Salter's  groups 
and  the  order  here  proposed  is  different. 

The  presence  of  a  part  homologous  with  the  free-cheeks  of 
other  trilobites  has  generally  been  more  or  less  overlooked 
in  the  families  of  this  order.  In  Trinucleus,  Dionide,  and 
Harpes  the  sutures  have  been  correctly  determined  by  Bar- 
rande.3  Likewise,  Angelin2  gave  the  right  structure  in 
Ampyx,  but  in  Agnostus  this  feature  has  escaped  notice. 
The  examination  of  extensive  series  of  Agnostus,  in  the 
National  Museum  and  in  the  Museum  of  Comparative  Zool- 
ogy,* has  proved  that  under  favorable  conditions  of  preserva- 
tion this  genus  shows  a  distinct  plate,  separated  from  the 
cranidium  by  a  suture,  and  it  can  be  compared  only  with  the 

*  In  the  former,  through  the  courtesy  of  C.  D.  Walcott  and  C.  Schuchert,  and 
in  the  latter,  of  A.  Agassiz  and  R.  T.  Jackson. 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    185 

free -cheeks  in  other  trilobites,  especially  where  they  are  con- 
tinuous around  the  front  of  the  cephalon,  as  in  Trinucleus 
and  Ampyx.  The  presence  of  a  hypostoma  in  Agnostus  was 
also  determined.  Even  in  the  higher  genera  of  this  order 
the  suture  is  frequently  unnoticed  in  descriptions,  but  it  can 
be  seen  in  all  well-preserved  specimens.  In  Trinucleus  29  and 
Harpes  it  follows  the  edge  of  the  cephalon,  and  separates 
the  dorsal  from  the  ventral  plate  of  the  pitted  limb.  Since 
eye-spots  occur  on  the  fixed-cheeks  in  the  young  Trinucleus 
and  adult  Harpes,  it  is  probable  that  this  character  is  a 
primitive  one  in  this  order,  and  has  been  lost  in  Agnostus, 
Microdiscus,  Ampyx,  and  Dionide. 

The  ontogeny  of  Sao,  Ptychoparia,  TriartJirus,  Dalmanites, 
etc.,  shows  that  the  true  eyes  and  free-cheeks  are  first  devel- 
oped ventrally,  appearing  later  at  the  margin,  and  then  on 
the  dorsal  side  of  the  cephalon.  Therefore  the  Agnostidas, 
Trinucleida?,  and  Harpedidse  have  a  very  primitive  head  struc- 
ture, characteristic  of  the  early  larval  forms  of  higher  families. 
Other  secondary  features  show  ,that  this  order,  though  the 
most  primitive  in  many  respects,  is  more  specialized  than 
either  of  the  others,  except  in  their  highest  genera.  The 
characters  referred  to  are  the  glabella  and  pygidium.  Very 
few  species  show  the  primitive  segmentation  of  the  glabella, 
it  being  usually  smooth  and  inflated,  and  resembling  in  its 
specialization  such  higher  genera  as  Proetus,  Asaphus,  and 
Lichas.  The  pygidium  often  fails  to  indicate  its  true  num- 
ber of  segments.  Some  Agnostus  and  Microdiscus  show  no 
segments  either  on  the  axis  or  limb  of  the  pygidium.  Trinu- 
cleus and  others  may  have  a  many-annulated  axis  and  fewer 
grooves  on  the  pleural  portions.  The  number  of  appendages 
corresponds  to  the  axial  divisions,  as  determined  by  the 
writer.4  The  multiplication  of  segments  in  the  pygidium  and 
their  consequent  crowding  makes  them  quite  rudimentary. 

Family  I.     AGNOSTIDJE  Dalman. 

Small  forms,  having  the  cephalon  and  pygidium  elongate, 
nearly  equal,  and  similar  in  form  and  markings.  Free-cheeks 


136  STUDIES  IN  EVOLUTION 

ventral,  continuous ;  suture  marginal  or  ventral.  Eyes  wanting. 
Thorax  composed  of  from  two  to  four  segments,  with  grooved 
pleura.  Cambrian  and  Ordovician. 

Including  the  genera  Agnostus  Brongniart  and  Microdiscus 
Emmons. 

The  genera  in  this  family  are  primitive  in  their  form  and 
structure,  as  shown  by  their  ventral  free-cheeks,  marginal 
or  ventral  suture,  elongate  cephalon,  and  large  pygidium. 
Some  species  have  spines  at  the  genal  angles,  corresponding 
to  the  interocular  spines  of  Holmia  and  young  Elliptocephala, 
and  not  to  the  spiniform  projections  of  the  free-cheeks. 
From  their  abbreviated  thorax  and  progressive  loss  of  annula- 
tions  on  the  glabella  and  axis  of  the  pygidium  they  must  also 
be  considered  as  degraded.  Microdiscus,  the  earlier  genus, 
has  three  or  four  free  segments,  and  in  some  species  (M.  spe- 
ciosus  Ford)  preserves  the  normal  pentamerous  glabella  and 
annulated  pygidial  axis,  while  the  later  genus,  Agnostus,  has 
but  two  free  segments,  and  has  lost  the  annulations  of  both 
glabella  and  pygidium.  Matthew26  has  described  the  pro- 
taspis  stage  of  Microdiscus,  which  agrees  with  the  similar 
stage  of  Ptychoparia  and  Sao. 

Fully  a  dozen  generic  names  have  been  proposed  for  forms 
of  the  general  type  of  Agnostus,  but  none  of  them  has  ever 
come  into  current  use.  Nine  were  first  published  by  Corda,15 
but  as  Barrande3  subsequently  showed  that  one  was  based 
on  an  Orbicula,  another  on  a  poor  specimen  of  ^ffiglina,  and 
three  others  on  a  single  species,  this  grouping  soon  fell  into 
disuse.  Moreover,  Barrande  was  inclined  to  give  no  generic 
value  to  the  form  and  lobation  of  the  glabella,  and  therefore 
all  the  species  were  placed  by  him  in  the  single  genus 
Agnostus.  At  the  present  time  more  weight  is  given  to  the 
characters  of  the  glabella  and  pygidium,  as  indicating  generic 
differences  in  dorsal  and  ventral  structure,  so  that  further 
study  may  show  the  desirability  of  restoring  such  of 
Corda' s  names  as  were  founded  upon  natural  groups  of  this 
family. 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    137 

Family  II.     HAKPEDID^E  Barrande. 

Cephalon  large,  margined  by  a  broad  expansion  or  limb; 
glabella  short  and  prominent.  Free-cheeks  ventral,  continu- 
ous; suture  marginal,  following  the  outer  edge  of  the  limb. 
Paired  simple  eye-spots,  or  ocelli,  single  or  double,  at  the  distal 
ends  of  well-marked  eye-lines  on  the  fixed-cheeks,  extending 
outward  from  the  glabella.  Thorax  of  from  twenty-five  to 
twenty-nine  segments,  with  long  grooved  pleura.  Pygidiurn  (in 
Harpes)  very  small,  composed  of  but  three  or  four  segments. 

Cambrian  to  Devonian. 

Including  Harpes  Goldfuss,  Harpina  Novak,  and  Harpides  ? 
Beyrich. 

The  genus  Harpes  presents  considerable  variation  in  the 
lobes  of  the  glabella.  H.  ungula  Steinberg  shows  the  full 
number  of  five  lobes,  but  in  some  species,  as  H.  d'Orlig- 
nyianum  Barrande,  the  structure  is  like  Cyphaspis,  with 
separate  basal  lobes.  Arraphus  Angelin  was  apparently 
based  upon  a  specimen  of  Harpes  denuded  of  the  pitted 
border.  Harpides  Beyrich  is  imperfectly  known,  but  seems 
to  belong  here.  The  ocular  ridges  and  tubercles  on  the 
fixed-cheeks,  the  broad  limb,  the  glabella,  and  the  narrow 
weak  thoracic  segments  are  all  in  accord  with  Harpes,  though 
in  other  features  it  has  affinities  with  the  Conocoryphidae. 

In  many  respects  Harpes  is  one  of  the  most  interesting 
genera  of  trilobites,  since  it  is  so  unlike  other  forms.  The 
broad  hippocrepian  pitted  limb  of  the  cephalon  has  its 
counterpart  in  Trinudeus  and  Dionide,  although  not  so  well 
developed  in  these  genera.  The  cephalon  is  also  comparatively 
longer  and  larger,  both  features  being  decidedly  larval.  It 
is  the  only  family  known  in  which  functional  visual  spots,  or 
ocelli,  are  situated  on  the  fixed-cheeks.  The  young  Trinu- 
deus has  similar  eye-spots,  or  ocelli.  The  great  number  of 
free  segments  in  the  Harpedidse  is  another  primitive  char- 
acter, although  the  cephalon  (in  Harpes)  still  remains  larger 
than  the  thorax  and  pygidium. 


138  STUDIES  IN  EVOLUTION 

Family  III.     TRINUCLEID^:  Barrande. 

Cephalon  larger  than  the  thorax  or  pygidium;  genal  angles 
produced  into  spines.  Free-cheeks  continuous,  almost  wholly 
ventral,  carrying  the  genal  spines;  suture  marginal  or  sub- 
marginal.  Paired  simple  eyes,  or  ocelli,  generally  absent  in 
adult  forms ;  compound  eyes  wanting.  Segments  of  thorax  five 
or  six  in  number,  with  grooved  pleura.  Pygidium  triangular ; 
margin  entire ;  axis  with  a  number  of  annulations ;  limb  grooved. 

Ordovician  and  Silurian. 

Including  the  genera  and  subgenera  Trinucleus  Lhwyd,  Ampyx 
Dalman,  Dionide  Barrande,  Endymionia  ?  Billings,  Lonchodomus 
Angelin,  Raphiophorus  Angelin,  and  Salteria  ?  W.  Thompson. 

The  leading  genera  of  this  family  form  a  tolerably  homoge- 
neous group,  although  each  has  sometimes  been  recognized 
as  characterizing  a  separate  family.  Trinucleus  and  Dionide 
have  a  broad  pitted  border,  but  this  hardly  seems  of  sufficient 
importance  to  remove  them  far  from  Ampyx,  since  the  three 
genera  agree  in  nearly  all  important  structural  details,  as 
the  extent  and  character  of  the  free  cheeks,  the  glabella,  the 
number  of  free  segments,  and  the  character  of  the  pygidium. 
Lonchodomus  and  Raphiophorus  of  Angelin  are  commonly 
admitted  as  sub-genera  of  Ampyx. 

Both  Salteria  W.  Thompson  and  Endymionia  Billings  have 
been  described  as  sub-genera  of  Dionide  Barrande,  though 
there  is  little  positive  evidence  for  this  disposition  of  them. 
Until  more  perfect  material  representing  these  forms  has 
been  described,  it  will  not  be  possible  to  decide  satisfactorily 
upon  their  relationships  or  place  in  a  classification.  There- 
fore they  are  left  with  doubt  in  the  present  family. 

Order  B.  OPISTHOPAEIA,  nov.  ord. 
(omo-dev  behind,  and  irapeia  cheek  piece.) 

Free-cheeks  generally  separate,  always  bearing  the  genal 
angles.  Facial  sutures  extending  forwards  from  the  posterior 
part  of  the  cephalon  within  the  genal  angles,  and  cutting  the 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    139 

anterior  margin  separately,  or  rarely  uniting  in  front  of  the 
glabella.  Compound  paired  holochroal  eyes  on  free-cheeks,  and 
well  developed  in  all  but  the  most  primitive  families. 

Including  the  families  Conocoryphidse,  Olenidse,  Asaphidse, 
Proetidse,  Bronteidse,  Lichadidee,  and  Acidaspidse. 

This  order  is  nearly  equivalent  to  group  B,  or  the  Asaphini 
of  S alter,  which  included  also  the  families  Calymmenidee  and 
Harpedidse,  which  belong  elsewhere. 

The  families  which  are  here  placed  under  this  order  lend 
themselves  quite  readily  to  an  arrangement  based  upon  the 
characters  successively  appearing  in  the  ontogeny  of  any  of 
the  higher  forms.  Thus  Sao,  Ptychoparia,  and  other  genera 
of  the  Olenidse  have  first  a  protaspis  stage  only  comparable 
in  the  structure  of  the  cephalon  with  the  genera  of  the  pre- 
ceding order,  the  Hypoparia.  Therefore  this  stage  does  not 
enter  into  consideration  in  an  arrangement  of  the  families 
of  the  Opisthoparia.  In  the  later  stages,  however,  there  is 
a  direct  agreement  of  structure  with  the  lower  genera  of 
this  order.  The  nepionic  Sao,  with  two  thoracic  segments 
(Plate  II,  figure  2),  has  a  head  structure  agreeing  in  essential 
features  with  that  in  Atops  or  Conocoryphe  (Plate  II,  figures 
14,  15).  A  later  nepionic  stage,  with  eight  thoracic  segments 
(Plate  II,  figure  3),  agrees  closely  with  adult  Ptychoparia  or 
Olenus  (figures  16,  17).  These  facts  clearly  indicate  that 
the  family  Conocoryphidse  should  be  put  at  the  base  of  this 
extensive  order.  Then,  as  Ptychoparia  and  Olenus  are  more 
primitive  and  simple  genera  than  Sao,  they,  as  typifying  the 
family  Olenidse,  should  govern  its  position,  which  accord- 
ingly would  be  next  after  the  Conocoryphidse.  In  each  case 
a  family  is  considered  as  represented  by  its  typical  and  most 
characteristic  forms.  It  would  be  impossible  to  consider  the 
advanced  specialized  genera  of  some  families  as  representing 
their  normal  facies,  for  each  one  has  undergone  an  indepen- 
dent evolution,  and  some  characters  have  reached  as  great 
a  degree  of  differentiation  as  will  be  found  in  much  higher 
families. 


140  STUDIES  IN  EVOLUTION 

It  has  been  recognized  that  variations  in  the  position  of 
the  eyes,  the  relative  size  of  the  free-  and  fixed-cheeks,  and 
the  degree  of  specialization  of  the  glabella  have  a  definite 
order  in  the  ontogeny  of  any  trilobite,  and  also  that  these 
characters  have  a  greater  taxonomic  value  than  many  others. 
Applying  these  principles  in  arranging  the  families  which 
come  under  the  Opisthoparia,  we  have  the  sequence  as  indi- 
cated above,  beginning  with  the  Conocoryphidae  and  followed 
by  the  Olenidse,  Asaphidse,  Proetidse,  Bronteidse,  Lichadidse, 
and  Acidaspidse,  in  regular  progression.  See  Plate  II. 
figures  14-23. 

Family  IV.     CONOCORYPHIDJE  Angelin. 

Free-cheeks  very  narrow,  forming  the  lateral  margins  of  the 
cephalon,  and  bearing  the  genal  spines.  Fixed-cheeks  large, 
usually  traversed  by  an  eye-line  extending  from  near  the  ante- 
rior end  of  the  glabella.  Facial  sutures  running  from  just  within 
the  genal  angles,  curving  forward,  and  cutting  the  anterior 
lateral  margins  of  the  cephalon.  Eyes  rudimentary  or  absent. 
Thorax  with  from  fourteen  to  seventeen  segments.  Pygidium 
small  and  of  few  segments.  Cambrian. 

Including  the  genera  and  sub-genera  Conocoryphe  Corda  (=  Co- 
nocephalites  Barrande),  Aneucanthus  Angelin,  Atops  Emmons, 
Avalonia  Walcott,  Bailiella,  Matthew  (=  Salteria  Walcott  and 
Erinnys  Salter),  Bathynotus  Hall,  Carausia  Hicks,  Carmon 
Barrande,  Ctenocephalus  Corda,  Dictyocephalites  Bergeron,  Eryx 
Angelin,  Harttia  Walcott,  and  Toxotis  Wallerius. 

The  genera  coming  under  this  family  present  a  number  of 
very  primitive  characters  such  as  are  shown  only  in  the  larval 
stages  of  higher  forms.  The  free-cheeks  are  narrow  and 
marginal,  and  can  be  compared  with  those  in  the  nepionic 
stages  of  Sao  and  PtycJioparia.  The  eyes  have  not  been 
detected,  but  the  presence  of  an  eye-line  suggests  their  pos- 
sible existence.  The  variations  of  the  glabella  are  very 
marked,  and  are  as  great  as  those  which  in  higher  forms 
attain  some  importance  as  family  characteristics.  In  Toxotis, 
Carausia,  and  Aneucanthus  the  glabella  expands  in  front, 


NATURAL   CLASSIFICATION   OF  THE  TRILOBITES    141 

joining  and  forming  part  of  the  anterior  margin,  as  in  the 
glabella  of  the  larval  stages  of  Solenopleura,  Liostracus, 
Ptychoparia,  and  Sao.  Ctenocephalus  and  Eryx  are  slightly 
more  advanced,  as  the  glabella  no  longer  marks  the  edge  of 
the  cephalon.  In  Atops,*  Avalonia,  Bathynotus,  and  Carmon 
the  glabella  is  cylindrical,  distinctly  defined,  and  limited 
within  the  margin,  and  in  Conocoryphe,  Harttia,  and  Bailiella 
it  narrows  anteriorly,  and  only  extends  about  two-thirds  the 
length  of  the  cephalon.  Generally  in  this  family  the  glabella 
displays  its  primitive  pentamerous  origin.  In  Bailiella  and 
Oarausia  two  basal  lobes  are  marked  off  from  the  fourth  seg- 
ment by  oblique  furrows,  as  in  Proetus  and  Cyphaspis. 

From  a  phylogenetic  standpoint  the  family  Conocoryphidse 
is  at  the  base  of  this  extensive  order.  As  far  as  known,  all 
the  larval  forms  in  the  other  families  of  the  Opisthoparia 
agree  in  having  the  narrow  marginal  free-cheeks,  bearing 
the  genal  angles.  The  eye-line  is  present  in  most  of  the 
adult  Olenidse,  and  in  the  early  stages  of  all  as  far  as  known, 
so  that  the  general  average  of  the  characters  in  the  Conoco- 
ryphidse represents  the  main  larval  features  throughout  the 
other  families.  They  show,  too,  that  although  primitive  in 
essential  structure,  differentiation  through  time  has  developed 
secondary  features  belonging  to  genera  in  higher  families;  as, 
for  example,  the  basal  glabellar  lobes  in  Bailiella. 


Family  V.     OLENID^  Salter. 

Cephalon  larger  than  the  pygidiuin,  usually  wider  than  long; 
genal  angles  commonly  produced  into  spines.  Free-cheeks  sepa- 
rate. Facial  sutures  extending  forward  from  the  posterior  mar- 
gin of  the  cephalon  along  the  eye-lobes,  and  either  cutting  the 
anterior  margin  separately  or  meeting  on  the  median  line.  Eyes 
crescentic,  reniform,  or  semi-circular,  situated  at  the  ends  of  eye- 
lines  in  all  but  highest  genera.  Trunk  long,  composed  of  from 

*  Atops  (type  A.  truineatus  Eramons)  seems  to  be  a  valid  genus,  and  differs 
from  Conocoryphe  (type  C.  Sulzeri  Schlotheim)  in  its  glabellar  characters,  greater 
number  of  thoracic  segments,  and  much  smaller  pygidium  with  fewer  segments. 


142  STUDIES  IN  EVOLUTION 

eight  (?)  to  twenty-six  free  segments ;  rarely  capable  of  rolling 
up.  Pygidium  frequently  small;  margin  entire  or  spinose. 

Principally  Cambrian,  but  extending  into  the  Ordovician. 

Including  the  genus  Olenus  Dalrnan  as  the  type,  and  the  fol- 
lowing genera  and  sub-genera,  which  should  doubtless  fall  into 
several  sub-family  or  even  family  groups:  Acerocare  Angelin, 
Acrocephalites  Wallerius,  Agraulus  Corda,  Angelina  Salter, 
Anomocare  Angelin,  Anopolenus  Salter,  Asaphelina  Bergeron, 
Bavarilla  Barrande,  Bergeronia  Matthew,  Boeckia  Brogger, 
Cer  atopy  ge  Corda,  Chariocephalus  Hall,  Corynexochus  Angelin, 
Crepicephalus  Owen,  Ctenopyge  Linnarsson,  Cyclognathus  Lin- 
narsson,  Dikelocephalus  Owen  (Centropleura  Angelin),  Dorypyge 
Dames,  Ellipsocephalus  Zenker,  Elliptocephala  Emmons,  Euloma 
Angelin,  Eurycare  Angelin,  Holmia  Matthew,  ffydrocephalus 
Barrande  (==.  young  Paradoxides),  Leptoplastus  Angelin,  Lios- 
tracus  Angelin,  Loganellus  Devine,  Menocephalus  Owen,  Mesona- 
cis  Walcott,  Micmacca  Matthew,  Neseuretus  Hicks,  Olenelloides 
Peach,  Olenellus  Hall,  Olenoides  Meek,  Oryctocephalus  Walcott, 
Palceopyge  Salter,  Parabolina  Salter,  Parabolinella  Brogger, 
Paradoxides  Brongniart,  Peltura  Angelin,  Plutonides  Hicks, 
Proceratopyge  Wallerius,  Protagraulus  Matthew,  Protolenus 
Matthew,  Protopeltura  Brogger,  Protypus  Walcott,  Pteroce- 
phalia  Koemer,  Ptychaspis  Hall,  Ptychoparia  Corda,  Remopleu- 
rides  Portlock,  Sao  Barrande,  Schmidtia  Marcou,  Solenopleura 
Angelin,  Sphceropthalmus  Angelin,  Telephus  Barrande,  Triar- 
thrella  Hall,  Triarthrus  Green,  and  Zacanthoides  Walcott. 

A  complete  study  of  this  extensive  family  of  trilobites 
would  contribute  much  in  the  way  of  generic  synonymy,  and 
bring  out  the  characters  necessary  for  family  determination 
and  subdivision.  This  important  work  must  be  left  for 
future  investigation.  So  many  genera  have  been  described 
from  separate  cranidia  or  even  pygidia  as  to  make  it  impos- 
sible to  deal  with  all  of  them  in  a  systematic  manner.  The 
zeal  to  make  the  most  out  of  the  earliest  known  faunas  has 
led  many  investigators  to  describe  and  recognize  imperfect 
and  poorly  preserved  material,  and  to  establish  genera  upon 
very  tenuous  characters.  Therefore,  without  a  most  inti- 
mate knowledge  of  all  the  forms,  any  grouping  of  the  major- 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    143 

ity  of  the  Cambrian  genera  into  families  or  the  limitations 
of  the  genera  themselves  must,  as  in  the  present  instance,  be 
taken  tentatively  and  as  necessarily  incomplete. 

A  number  of  genera  have  been  already  made  the  types  of 
family  divisions;  as  Paradoxides,  Olenellus,  Remopleurides, 
Ellipsoceplialus,  Ptychoparia,  etc.  Some  of  them  may  be 
shown  ultimately  to  possess  characters  of  sufficient  weight 
to  be  entitled  to  family  distinction.  A  preliminary  grouping 
of  the  best-known  genera  may  be  of  some  value  here,  and 
for  the  sake  of  convenience  these  divisions  may  be  defined 
as  sub-families.  Four  groups  will  be  recognized,  of  which 
Paradoxides,  Oryctocephalus,  Olenus,  and  Dikelocephalus  are 
taken  as  representative  genera. 

I.  Paradoxinse.  —  Including  Olenellus,  Holmia,  Mesonacis, 
Elliptocephala,  Schmidtia,  Ohnelloides,  Paradoxides,  Zacan- 
thoides,  and  Remopleurides.  Most  of  the  genera  are  distin- 
guished by  their  long  narrow  eyes,  often  extending  more 
than  half  the  length  of  the  glabella,  but  more  especially  by 
the  rudimentary  character  of  the  pygidium.  In  Olenellus 
the  pygidium  is  a  long  telson-like  spine.  In  Holmia,  Meso- 
nacis, Elliptocephala,  and  Schmidtia  it  is  reduced  to  a  small 
plate  without  distinct  segmental  divisions.  In  Paradoxides, 
Zacanthoides,  and  Remopleurides  the  axis  may  show  from  one 
to  five  annulations,  while  the  limb  may  carry  two  or  three 
pairs  of  spines  or  may  be  entire.  In  Olenellus  and  Holmia 
true  facial  sutures  have  been  denied  by  some  authors,  but  in 
their  place  false  sutures  are  recognized.  They  are,  however, 
evidently  real  sutures  in  a  condition  of  symphysis,  which 
often  occurs  in  Phacops,  Proe'tus,  Pliillipsia,  etc.  Otherwise 
these  genera  would  violate  the  first  principle  of  trilobite 
structure,  in  not  having  the  compound  eyes  on  the  free- 
cheek  pieces.  Olenelloides  is  a  very  striking  form,  but  its 
pygidium  is  unknown,  and  the  head  structure  is  obscure. 
The  elongate  cephalon  is  a  decidedly  larval  feature,  and  the 
genal  and  interocular  (?)  spines  strongly  suggest  its  immature 
condition,  and  point  to  the  possibility  of  its  being  the  young 
of  Olenellus  or  a  related  form. 


144  STUDIES  IN  EVOLUTION 

There  has  been  much  discussion  as  to  the  synonymy  and 
value  of  most  of  the  names  proposed  as  genera  or  sub-genera 
in  this  group.  Paradoxides,  Remopleurides,  and  Zacanthoides 
are  about  the  only  ones  that  have  escaped  severe  criticism  in 
recent  years.  Taking  the  type  of  each  of  the  others,  it  is 
found  that  Elliptocephala  (1844)  was  based  on  the  species 
E.  asaphoides  Emmons,  Olenellus  (1862)  on  0.  Thompsoni 
Hall,  Mesonacis  (1885)  on  M.  vermontana  Hall  sp.,  Holmia 
(1890)  on  R.  Kjerulfi  Linnarsson  sp.,  Schmidtia  (1890)  on 
S.  Mickwitzi  Schmidt  sp.,  and  Olenelloides  (1894)  on  0.  arma- 
tus  Peach.  Some  of  these  names  are  generally  recognized 
as  sub- genera  of  Olenellus  (Mesonacis,  Holmia,  Olenelloides), 
while  others  are  considered  as  synonyms  (Elliptocephala, 
Schmidtia).  The  early  genera  were  described  from  very  in- 
complete material,  and  therefore  lacked  sufficient  diagnostic 
characters  to  define  them  clearly.  At  the  present  time 
nearly  or  quite  entire  specimens  representing  the  type  species 
are  known,  and  it  is  possible  to  compare  all  the  essential 
features  with  some  degree  of  accuracy.  The  main  characters 
offering  the  greatest  variation  are  (1)  the  number  of  thoracic 
segments  and  (2)  their  specialization  into  groups,  (3)  the 
relative  development  of  the  third  free  segment,  (4)  the  num- 
ber and  position  of  the  spine -bearing  segments,  (5)  the  form 
of  the  pygidium,  (6)  the  presence  or  absence  of  interocular 
spines,  and  (7)  the  form  of  the  cephalon.  A  simple  varia- 
tion in  any  one  of  these  would  not  necessarily  imply  more 
than  a  specific  difference,  but  the  genera  here  mentioned 
exhibit  marked  changes  in  all  or  nearly  all  of  these  charac- 
ters, and  in  any  family  should  receive  recognition.  Olenellus, 
Mesonacis,  and  Elliptocephala  are  more  closely  related  than 
the  other  forms,  and  probably  have  only  a  sub-generic  value 
under  Elliptocephala.  In  the  first  form  with  fourteen  thoracic 
segments,  the  third  is  greatly  enlarged  and  the  fifteenth  is 
the  spiniform  telson-like  pygidium.  In  Mesonacis  with 
twenty-six  thoracic  segments,  the  third  is  somewhat  en- 
larged, and  behind  the  narrow  spine-bearing  fifteenth  seg- 
ment there  are  eleven  others  without  spines,  followed  by  the 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    145 

small  plate-like  pygidium.  In  Elliptocephala  with  eighteen 
thoracic  segments,  the  cephalon  is  broader,  the  third  segment 
is  not  enlarged  except  in  the  young,  and  the  fourteenth  to 
eighteenth  segments  are  narrower  and  spine-bearing. 

II.  Oryctocephalinae.  —  Including   Oryctocephalus,  Cteno- 
pyge,  Olenoides,  and  Parabolina,  with  large  pygidia  and  all 
but   the   last  one   or  two  pleural   elements  continued  into 
spines;   also  Eurycare,  Angelina,  Peltura^  and  Protopeltura, 
with  smaller  and  shorter  pygidia  and  denticulations  of  the 
margins  corresponding  to  the  pleural  divisions. 

III.  Oleninee.  —  Including   Olenus,  Agraulus,  Liostracus, 
Acerocare,  Ptychoparia,  Solenopleura,  PtycJiaspis,  Leptoplas- 
tus,  Loganellus,  Sphceropthalmus,  Parabolinella^  Boeckia,  Pro- 
ceratopyge,  Ceratopyge,  Protypus,  Ellipsocephalus,  Sao,  and 
Triarthrus.     All  these  genera  have  small  or  medium-sized 
pygidia,  with   from   two   to   eight  annulations  in  the  axis. 
Eyes  medium  to  small,  at  the  ends  of  distinct  eye-lines  in 
all  but  the  latest  genera,  which  preserve  this  character  only 
during  the  young  stages.     Thoracic  segments  from  eleven  to 
eighteen. 

IV.  Dikelocephalinse.  —  Including  Dikelocephalm,  Asaphe- 
lina,  and  Crepicephalus.     Eight  or  nine  thoracic  segments. 
Pygidium  wide,  with  the  posterior  lateral  portion  often  pro- 
duced into  broad  spine-like  extensions.     Dikelocephalus  is  in 
many  ways  related  to  Ogygia  and  Asaphus. 

Family  VI.     ASAPHID^B  Emmrich. 

Cephalon  and  pygidium  well  developed;  glabella  often  ob- 
scurely limited.  Free-cheeks  usually  separate.  Facial  sutures 
extending  forward  from  the  posterior  edge  of  the  cephalon 
within  the  genal  angles,  and  cutting  the  lateral  or  anterior 
margins,  occasionally  uniting  in  front  of  the  glabella.  Eyes 
smooth,  well  developed,  sometimes  of  very  large  size,  even 
occupying  the  entire  surface  of  the  free-cheeks.  Thorax  gener- 
ally composed  of  eight  or  ten  segments,  but  varying  from  five 
to  ten;  capable  of  enrolment.  Pygidium  large,  often  with 
wide  doublure.  Cambrian,  Ordovician,  and  Silurian. 

10 


146  STUDIES  IN  EVOLUTION 

The  long  list  of  genera  in  this  family  may  be  easily  divided 
into  two  sections,  which  are  often  recognized  as  of  family  rank. 

I.  ASAPHID^E.  —  Including    the    genera   and   sub-genera 
Asaphus  Brongniart  (=  Cryptonymus  Eichwald),  Asaphellus 
Callaway,   Asaphiscus   Meek,    Barrandia   McCoy,    Basilicus 
Salter,  Bathyurellus  Billings,  Bathyuriscus  Meek,  Bathyurus 
Billings,  Bolbocephalus  Whitfield,  Brachyaspis  Salter,  Bron- 
teopsis  W.  Thompson,  Dolichometopus  Angelin,    Geramphes 
Clarke,  Holasaphus  Matthew,  Homalopecten  Salter,  Isotelus 
DeKay,   Megalaspides   Brogger,    Megalaspis   Angelin,   Niobe 
Angelin,  Ogygia  Brongniart,   Ogygiopsis  Walcott,  Phillipsi- 
nella  Novak,  Platypeltis  Callaway,  PtycTiopyge  Angelin,  and 
Stygina  Salter. 

This  is  a  tolerably  homogeneous  group,  although  some  of 
the  Cambrian  forms  have  a  sufficiently  archaic  expression 
to  make  them  seem  a  little  out  of  place  with  genera  of  so 
pronounced  a  family  type  as  Asaphus,  Niobe,  Ptychopyge, 
Megalaspis,  and  Isotelus. 

The  elements  of  the  glabella  are  generally  quite  obscure, 
and  even  its  limits  cannot  be  clearly  made  out  in  late  genera, 
as  Stygina  and  Asaphus.  The  segmental  nature  of  the  gla- 
bella is  clearly  shown  in  Ogygia,  Ogygiopsis,  Homalopecten, 
Asaphellus,  Bronteopsis,  and  Bathyuriscus. 

The  elements  of  the  pygidium  are  obscurely  marked  in 
Brachyaspis  and  Isotelus.  Phillipsinella  is  a  very  small 
form,  and  probably  the  young  of  an  Asaphus.  Barrandia, 
Homalopecten,  and  Stygina  serve  as  transition  genera  to  the 
Illsenidse. 

II.  ILL^ENHXE.  —  Including   the  genera  and  sub-genera 
Ulcenus   Dalman,    ^glina   Barrande     (=  Oyclopyge   Corda), 
Buwastus    Murchison,     Dysplanus    Burmeister,     Ectillcenus 
Salter,  Holocephalina   Salter,  Hydrolenus   Salter,    Illcenopsis 
Salter,   Hlcenurus   Hall,   Nileus  Dalman,    Octillcenus   Salter, 
Panderia  Volborth,  Psilocephalus  Salter,  Symphysurus  Gold- 
fuss,  and  Thaleops  Conrad. 

The  Illsenidse  form  a  much  more  compact  group  than  the 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    147 

preceding,  characterized  by  having  a  rostral  plate  and  by 
the  very  tumid  form  of  the  large  cephalon  and  the  obscure 
or  obsolete  boundaries  of  the  glabella  and  occipital  lobe. 
The  pygidium  often  closely  resembles  the  cephalon  in  size 
and  form,  and  the  axis  is  frequently  scarcely  denned. 

Considerable  variation  is  shown  in  the  size,  position,  and 
direction  of  the  visual  surfaces.  There  is  also  a  ratio  be- 
tween the  size  of  the  fixed-cheeks  and  the  eyes.  In  propor- 
tion as  the  fixed-cheeks  are  large,  the  eyes  are  small,  and  as 
the  area  of  the  fixed-cheeks  diminishes  from  a  widening  of 
the  axis  of  the  animal,  the  eyes  become  larger.  Thus,  in 
Holocephalina,  with  extremely  large  fixed-cheeks  and  narrow 
axis,  the  eyes  are  quite  small.  In  Hlcenopsis,  Dysplanus, 
Panderia,  and  Octillcenus  they  are  progressively  larger,  and 
in  Illcenus,  Bumastus,  and  Nileus,  where  the  axis  is  wide  and 
the  fixed-cheeks  are  reduced,  the  eyes  are  relatively  large. 
This  variation  reaches  its  limit  in  the  species  of  jtEglina, 
where  the  axis  is  very  wide  and  the  fixed-cheeks  are  reduced 
to  almost  nothing,  so  that  the  glabella  and  eyes  make  up  the 
entire  dorsal  surface  of  the  cephalon.  In  ^glina  princeps 
Barrande  the  eyes  extend  about  half  the  length  of  the  cepha- 
lon. The  eyes  of  ^E.  rediviva  Barrande  bound  the  whole 
length  of  the  sides  of  the  head,  and  in  jffl.  armata  Barrande 
the  coalesced  free-cheek  pieces  are  almost  wholly  converted 
into  a  visual  area,  so  that  there  is  a  continuous  eye  around 
the  sides  and  front  of  the  cephalon. 

Variations  in  the  position  of  the  eyes  are  to  be  noted  in 
nearly  all  the  genera.  In  Ectillcenus  and  Psilocephalus  they 
are  in  front  of  the  middle  of  the  length  of  the  cephalon,  and 
in  Dysplanus,  Illcenopsis,  and  Holocephalina  they  are  near  the 
posterior  angles  of  the  cranidium.  Panderia  has  the  eyes 
directed  obliquely  backward,  and  in  Thaleops  they  are  carried 
on  conical  extensions  pointing  outward. 

Family  VII.     PROETID^E  Barrande. 

Cephalon  about  one-third  of  the  whole  animal;  genal  angles 
generally  produced  into  spines;  glabella  tumid,  with  two  lateral 


148  STUDIES  IN  EVOLUTION 

basal  lobes  defined  by  oblique  furrows  in  front  of  the  neck  seg- 
ment. Free-cheeks  large,  separate.  Sutures  extending  from 
the  posterior  margin  inward  to  the  eyes,  and  then  forward,  cut- 
ting the  anterior  margins  separately.  Eyes  prominent,  often 
large.  Thorax  of  from  eight  to  twenty-two  free  segments,  with 
grooved  pleura.  Pygidium  usually  of  many  segments;  pleural 
and  axial  portions  strongly  grooved;  margin  entire  or  dentate. 

Ordovician  to  Permian. 

Including  the  genera  and  sub-genera  Proetus  Steininger, 
Arethusina  Barrande,  Brachymetopus  McCoy,  Celmus  Angelin, 
Cordania  Clarke,  Crotalurus  Volborth,  Cyphaspis  Burmeister, 
Dechenella  Kayser,  Griffithides  Portlock,  Phaetonella  Novak, 
Phillipsia  Portlock,  Prionopeltis  Corda,  Pseudophillipsia  Gem- 
mellaro,  Schmidtella  Tschernyschew,  Tropidocoryphe  Novak,  and 
Xiphogomium  Corda. 

The  genera  of  this  family  readily  fall  into  a  series  express- 
ing more  or  less  closely  the  development  and  specialization 
of  various  characters.  Arethusina  is  the  only  genus  retaining 
the  archaic  eye-lines,  and  both  on  this  account  and  for  the 
comparatively  forward  position  of  the  eyes  (itself  a  nepionic 
character),  together  with  the  large  number  of  thoracic  seg- 
ments, it  stands  near  the  base  of  the  series. 

The  eyes  gradually  approach  the  axis,  and  move  back- 
ward through  the  genera  Tropidocoryphe,  Cyphaspis,  Proetus, 
Prionopeltis,  Phillipsia,  and  Grriffithides.  Concurrent  with  this 
variation,  there  is  a  reduction  of  the  fixed-cheeks  and  exten- 
sion of  the  glabella.  In  Arethusina,  Tropidocoryphe,  Cor- 
dania, and  Cyphaspis  the  fixed-cheeks  are  about  the  size  of 
the  free -cheeks,  and  occupy  a  large  portion  of  the  cranidium. 
They  are  more  reduced  in  Proetus  and  Prionopeltis,  and  in 
Phillipsia  and  Grriffithides  they  form  only  a  narrow  border 
to  the  glabella.  The  lobation  of  the  glabella  varies  greatly, 
and  few  species  retain  evidences  of  its  original  segmental 
nature.  Some  Proetus  and  Dechenella  show  this  feature,  but 
in  many  Phillipsia  and  Griffithides  the  elements  cannot  be 
made  out.  In  Proetus  there  is  often  a  small  accessory  lobe 
developed  at  the  ends  of  the  neck  ring,  which  is  only  of 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    149 

interest  as  being  homologous  with  similar  lobes  in  many 
of  the  Lichadidse  and  Acidaspidse,  where  they  often  become 
very  conspicuous.  In  all  the  Proetidse  the  oblique  lobes  of 
the  fourth  annulus  of  the  glabella  are  also  important  in  this 
connection,  as  here  again  is  marked  the  inception  of  side 
axial  lobes,  which  become  prominent  features  in  higher 
genera,  indicating  greater  specialization  of  the  organs  and 
appendages  of  the  head. 

Family  VIII.     BRONTEID^E  Barrande. 

Dorsal  shield  broadly  elliptical.  Cephalon  less  than  one- 
third  the  entire  length;  glabella  rapidly  expanding  in  front, 
with  faint  indications  of  lobes.  Free-cheeks  larger  than  the 
fixed-cheeks.  Facial  sutures  extending  from  the  posterior  mar- 
gin just  behind  the  eyes  abruptly  inward  around  the  palpebral 
lobes,  and  then  diverging  and  cutting  the  antero-lateral  margins 
separately.  Eyes  crescentic.  Thorax  of  ten  segments,  with 
ridged  pleura.  Pygidium  longer  than  cephalon  or  thorax;  axis 
very  short,  with  radiating  furrows*  extending  from  it  across  the 
broad  limb  toward  the  margin;  doublure  very  wide;  margin 
generally  entire.  Ordovician  to  Devonian. 

Including  the  single  genus  Bronteus  Goldfuss  (—  Goldius  de 
Koninck). 

Many  of  the  species  of  Bronteus  (as  B.  angusticeps 
Barrande,  B.  palifer  Beyrich)  show  a  breaking  up  of  the 
glabella  into  symmetrically  disposed  separate  lobes,  as  in 
Conoliclias  and  Acidaspis.  The  frontal  lobe  is  transverse  and 
much  larger  than  the  others.  Back  of  it  may  be  simple 
grooves  marking  the  elements  (#.  campanifer  Beyrich),  or 
there  may  be  one  or  two  circular  or  elliptical  swellings  on 
each  side  of  the  axis  (B.  angusticeps  Barrande),  or,  in  addition, 
the  axial  portion  may  consist  of  several  lobes.  The  reduction 
of  the  axis  of  the  pygidium  and  the  expansion  of  the  limb 
meet  with  their  greatest  expression  in  this  genus.  Lichas 
shows  the  decline  of  these  characters,  the  pygidial  limb 
becoming  more  or  less  deeply  lobed,  and  finally  the  lobes  are 


150  STUDIES  IN  EVOLUTION 

represented  by  spines  (Arges,  Terataspis).     Further  progres- 
sion of  these  changes  is  shown  in  Acidaspis. 


Family  IX.     LICHADIDJE  Barrande. 

Dorsal  shield  generally  large  and  flat,  with  granulated  test. 
Cephalon  small,  not  more  than  one-fourth  the  entire  length; 
genal  angles  spiniform.  Free-cheeks  separate;  sutures  extend- 
ing from  the  posterior  margin  obliquely  inward  to  the  eyes,  and 
then  almost  directly  forward,  cutting  the  margin  separately. 
Glabella  broad,  with  a  large,  often  tumid,  central  lobe  and  from 
one  to  three  side  lobes.  Eyes  not  large.  Thorax  with  nine  or 
ten  segments  and  grooved  and  falcate  pleura.  Pygidium  large, 
flat,  commonly  with  toothed  or  notched  margin  corresponding 
to  the  pleural  grooves;  doublure  very  broad. 

Ordovician  to  Devonian. 

Including  the  genera  and  sub-genera  Lichas  Dalman,  Arcti- 
nurus  Castelnau,  Arges  Goldfuss,  Ceratolichas  Hall  and  Clarke, 
Conolichas  Dames,  Dicranogmus  Corda,  Homolichas  Schmidt, 
Hoplolichas  Dames,  Leiolichas  Schmidt,  Metopias  Eichwald, 
Oncholichas  Schmidt,  Platymetopus  Angelin,  Terataspis  Hall, 
Trochurus  Beyrich,  and  Uralichas  Delgado. 

Most  of  the  forms  of  this  family  are  above  the  average  size 
of  trilobites,  and  several  species  are  among  the  largest  of  the 
class.  They  are  all  thin-shelled,  and  were  loosely  articu- 
lated, so  that  entire  specimens  are  extremely  rare. 

A  great  diversity  is  shown  in  the  form  and  lobation  of  the 
glabella.  In  Lichas  (sens,  str.),  Platymetopus,  and  Leiolichas 
the  anterior  lobe  dominates  and  is  continuous  with  the  axis. 
In  Hoplolichas  and  Homolichas  the  lateral  lobes  are  strongly 
defined,  and  each  is  nearly  equal  in  size  to  the  central  lobe. 
Dicranogmus,  Oncholichas,  Conolichas,  Metopias,  Arctinurus, 
and  Arges  show  the  lateral  lobes  divided  transversely  into 
two  or  three  smaller  ones.  Lastly  in  Ceratolichas,  and  more 
especially  in  Terataspis,  the  central  lobe  becomes  a  promi- 
nent ovoid  or  globular  extension.  These  variations  evidently 
indicate  differences  in  the  relative  development  of  the  append- 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    151 

ages  and  organs  of  the  head,  and  therefore  are  of  consider- 
able morphological  importance. 

The  pygidium  is  composed  of  few  distinct  segments.  The 
annulated  portion  of  the  axis  is  generally  short,  and  the  den- 
tations on  the  border  of  the  limb,  corresponding  to  the 
pleural  grooves,  range  from  two  to  four  on  each  side.  Leio- 
lichas  is  the  only  form  which  has  an  entire  pygidial  margin. 

Family  X.     ACIDASPIDJE  Barrande. 

Dorsal  shield  spinose.  Cephalon  transversely  semi-elliptical, 
quadrate,  or  trapezoidal ;  genal  angles  spiniform.  Glabella  with 
one  large  median  axial  lobe  and  two  or  three  lateral  lobes. 
Free-cheeks  large,  separate.  Sutures  extending  from  within 
the  genal  angles  abruptly  inward  to  the  eyes,  and  then  forward, 
cutting  the  anterior  margin  each  side  of  the  glabella.  Eyes 
small,  often  prominent.  Thorax  of  eight  to  ten  segments,  with 
ridged  pleura  extended  into  hollow  spines.  Pygidium  usually 
small,  with  spinous  margin.  Ordovician  to  Devonian. 

Including  the  genera  and  sub-genera  Acidaspis  Murchison, 
Ancyropyge  Clarke,  Ceratocephala  Warder,  Dicranurus  Conrad, 
Odontopleura  Emmrich,  and  Selenopeltis  Corda. 

In  this  family  and  the  Lichadidse  is  shown  the  highest 
expression  of  differentiation  and  specialization  of  the  Opis- 
thoparia.  The  primitive  pentamerous  lobation  of  the  axis  of 
the  cranidium  is  entirely  obscured,  and  is  only  clearly  seen 
in  the  protaspis  and  early  nepionic  stages.  These  two  fam- 
ilies are  very  closely  related,  the  chief  differences  being 
noticed  in  the  size  and  character  of  the  pygidium,  and  the 
ribbed  or  grooved  pleura.  The  Lichades  are  generally  much 
larger  and  flatter,  but  the  smaller  and  highly  spinose  forms 
of  Arges,  Ceratolichas,  and  HoplolicJias  approach  quite  near 
some  of  the  Acidaspidse. 

It  has  been  customary  of  late  years  to  regard  all  the  species 
of  this  family  as  belonging  to  the  single  genus  Acidaspis,  and 
to  consider  the  various  sub-divisions  bearing  separate  names 
as  of  the  value  of  sub-genera.  Clarke  H  has  shown  that,  on 


152  STUDIES  IN  EVOLUTION 

the  basis  of  priority,  Oeratocephala  is  the  first  distinctive 
name  ever  applied  to  the  group,  and  is  therefore  entitled 
to  full  generic  recognition.  He  further  recognizes  Odonto- 
pleura,  Acidaspis,  Dicranurus,  Selenopeltis,  and  Ancyropyge  in 
the  sub-ordinate  position  of  sub-genera  under  Oeratocephala. 


Order  C.     PKOPAKIA,  nov.  ord. 
(TT/H)  before,  and  irapeid  cheek  piece.) 

Free-cheeks  not  bearing  the  genal  angles.  Facial  sutures 
extending  from  the  lateral  margins  of  the  cephalon  in  front 
of  the  genal  angles,  inward  and  forward,  cutting  the  anterior 
margin  separately  or  uniting  in  front  of  the  glabella.  Com- 
pound paired  eyes  scarcely  developed  or  sometimes  absent  in 
the  most  primitive  family,  well-developed  and  schizochroal  in 
last  family. 

Including  the  families  Encrinuridse,  Calymmenidae,  Cheiru- 
ridae,  and  Phacopidae. 

Salter's  first  division,  Phacopini,  included  the  two  families 
Phacopidse  and  Cheiruridse.  The  Calymmenidse  were  placed 
in  his  second  division,  the  Asaphini. 

This  is  the  only  order  of  trilobites  which  apparently  begins 
within  the  known  Paleozoic,  and,  unlike  the  other  orders,  it 
had  no  pre-Cambrian  existence.  The  earliest  forms  of  the 
Proparia  came  at  the  close  of  the  Cambrian,  in  the  lower 
Ordovician.  Its  greatest  generic  differentiation  was  attained 
early.  There  is  a  rapid  decline  in  the  Silurian  and  Devo- 
nian, and  only  one  or  two  genera  extend  to  the  beginning  of 
the  Carboniferous. 

In  the  Opisthoparia  it  was  demonstrated  that  the  Cono- 
coryphidse  formed  the  natural  base  or  most  primitive  family 
in  the  order,  and  was  distinguished  by  the  narrow  marginal 
free-cheeks  and  the  absence  of  well-developed  eyes.  It  is 
of  much  interest  and  importance  to  be  able  to  recognize,  in 
the  Proparia,  a  similar  primitive  family  having  characters  in 
common  with  the  former,  but  still  clearly  belonging  to  the 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    153 

higher  order.  Placoparia,  Areia,  and  Dindymene,  of  the 
Encrinuridse,  constitute  a  group  of  apparently  blind  trilo- 
bites,  with  narrow  marginal  free -cheeks,  presenting  in  gen- 
eral the  appearance  of  Atops,  Conocoryphe,  Ctenocephalus, 
etc.,  of  the  Conocoryphidse.  The  somewhat  higher  genera 
Cybele  and  Encrinurus  have  intermediate  or  transitional 
characters  leading  to  the  other  families.  The  Cheiruridaa 
show  a  greater  amount  of  differentiation  and  progressive  and 
regressive  evolution  than  any  other  in  this  order.  Crotalo- 
cephalus  arid  Sphcerexochus  seem  to  express  the  highest  de- 
velopment, and  Deiphon  and  Onycopyge  show  the  effects  of 
over-specialization,  resulting  in  degeneration.  The  Calym- 
menidse,  in  their  small  eyes  and  narrow  free-cheeks,  have 
decided  affinities  with  the  lower  genera.  The  same  may  be 
said  of  Trimerocephalus  of  the  Phacopidse,  though  the  other 
genera  of  this  family  possess  large  eyes,  situated  well  back 
and  close  to  the  glabella.  For  these  and  other  reasons,  the 
family  is  placed  at  the  end  of  the  order,  as  expressing  its 
highest  development. 

Family  XI.     ENCRINURIDSE  Linnarsson. 

Cephalon  narrow,  transverse.  Fixed-cheeks  very  large.  Free- 
cheeks  long,  narrow,  separate,  sometimes  with  a  free  plate 
between  the  anterior  extremities.  Sutures  extending  from  in 
front  of  the  genal  angles  obliquely  forward,  and  cutting  the 
anterior  margin  in  front  of  the  glabella.  Eyes  very  small  or 
absent.  Thorax  of  from  nine  to  twelve  segments,  with  ridged 
pleura.  Pygidium  generally  composed  of  many  segments;  limb 
with  strong  ribs  usually  less  in  number  than  the  annulations  of 
the  axis.  Ordovician  and  Silurian. 

Including  the  genera  Encrinurus  Emmrich  (Cromus  Barrande), 
Areia  Barrande,  Cybele  Loven,  Dindymene  Corda,  Placoparia 
Corda,  and  Prosopiscus  Salter. 

The  ConocoryphidaB  were  shown  to  be  the  radical  of  the 
order  Opisthoparia,  and  for  similar  reasons  the  EncrinuridaB 
may  now  be  taken  as  the  primitive  family  of  the  Proparia. 


154  STUDIES  IN  EVOLUTION 

The  cephala  of  Areia  and  Placoparia  have  many  resemblances 
to  Conocoryphe,  but  the  fixed-cheeks  bear  the  genal  angles 
and  spines,  while  in  the  latter  genus  they  are  on  the  free- 
cheeks.  In  both  families  the  free-cheeks  are  narrow  and 
marginal,  and  the  eyes  are  absent  or  rudimentary.  Both 
these  characters  are  decidedly  larval.  Other  primitive  and 
larval  features  belonging  to  the  Encrinuridse  are  the  eye- 
lines  in  Cybele  and  Encrinurus,  the  undefined  and  expanded 
termination  of  the  glabella  in  Dindymene  and  Encrinurus, 
and  the  pentamerous  head  axis  in  all  but  Dindymene,  in 
which  the  four  anterior  lobes  or  annulations  are  obsolete.  In 
Encrinurus  the  eye-line  in  meeting  and  joining  the  anterior 
lobe  of  the  glabella  sometimes  gives  the  appearance  of  an 
extra  lobe,  as  in  Ogygia  and  Paradoxides. 

Family  XII.     CALYMMENIDJS  Brongniart. 

Cephalon  somewhat  wider  than  long.  Fixed-cheeks  large; 
genal  angles  rounded  or  produced  into  spines.  Glabella  nar- 
rowing anteriorly.  Free-cheeks  long,  separate,  usually  with 
a  free  rostral  plate  between  the  anterior  extremities.  Sutures 
extending  from  just  in  front  of  the  genal  angles,  converging 
anteriorly,  and  cutting  the  margins  separately.  Eyes  small; 
visual  surface  seldom  preserved.  Thorax  of  thirteen  segments, 
with  grooved  pleura.  Pygidium  of  from  six  to  fourteen  seg- 
ments; axis  tapering.  Ordovician  to  Devonian. 

Including  the  genera  and  sub-genera  Calymmene  Brongniart, 
Brongniart ia  Salter,  Burmeisteria  Salter,  Calymmenella  Ber- 
geron, Calymmenopsis  Munier-Chalmas  and  Bergeron,  Dipleura 
Green,  Homalonotus  Koenig,  Koenigia  (=  Homalonotus)  Salter, 
Pharostoma  Corda,  Plcesiacomia  Corda,  Ptychometopus  Schmidt, 
and  Trimerus  Green. 

The  genera  of  this  family  naturally  cluster  around  the  two 
leading  ones,  Calymmene  and  Homalonotus.  Closely  related  to 
the  first  are  Ptychometopus,  Pharostoma,  Calymmenopsis,  and 
Calymmenella,  all  agreeing  in  having  the  glabella  well  defined 
and  marked  by  furrows  or  indentations  at  the  sides,  corre- 
sponding to  its  segmental  nature. 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    155 

The  second  group,  including  Brongniartia,  Trimerus, 
Homalonotus  (sens,  str.),  Plcesiacomia,  Dipleura,  and  Bur- 
meisteria,  agree  in  having  a  low,  not  sharply  defined,  quad- 
rate glabella,  without  distinct  furrows  or  lobes.  In  general, 
the  axis  of  the  thorax  and  pygidium  is  much  wider  than  in 
the  first  group,  and  the  pygidium  is  more  elongate  and  often 
pointed. 

Family  XIII.     CHEIRURID^E  Salter. 

Glabella  well  defined.  Free-cheeks  small,  sometimes  much 
reduced.  Sutures  extending  from  in  front  of  the  genal  angles 
inward  to  the  eyes,  and  then  obliquely  forward,  cutting  the 
anterior  margin  in  front  and  each  side  of  the  glabella.  Eyes 
usually  small.  Thorax  composed  of  from  nine  to  eighteen  seg- 
ments, generally  eleven;  pleura  often  extended  into  hollow 
spines.  Pygidium  small,  with  from  three  to  five  segments; 
pleural  elements  commonly  produced  into  spines. 

Principally  Ordovician  and  Silurian,  but  extending  into  the 
Devonian. 

Including  the  genera  and  sub-genera  Cheirurus  Beyrich,  Actin- 
opeltis  Corda,  Amphion  Pander,  Anacheirurus  Heed,  Ceraurus 
Green,  Crotalocephalus  Salter,  Cyrtometopus  Angel  in,  Deiphon 
Barrande,  Diaphanometopus  Schmidt,  Eccoptocheile  Corda,  Hem- 
isphcerocoryphe  Reed,  Nieszkowskia  Schmidt,  Onycopyge  Wood- 
ward, Pseudosphcerexochus  Schmidt,  Sphcerexochus  Beyrich, 
Sphcerocoryphe  Angelin,  Staurocephalus  Barrande,  and  Youngia 
Lindstrom. 

As  in  other  families,  the  most  primitive  genera  are  those 
in  which  the  regular  pentamerous  lobation  of  the  glabella  is 
retained,  with  the  eyes  well  forward,  the  free-cheeks  narrow, 
and  the  fixed-cheeks  ample.  Diaphanometopus,  Anacheiru- 
rus,  Eccoptocheile,  and  Cyrtometopus  agree  in  these  respects, 
and  therefore  belong  at  the  beginning  of  a  phylogenetic  list. 
Ceraurus  and  Nieszkowskia  appear  to  branch  off  here,  being 
characterized  by  the  narrow  transverse  form  of  the  cephalon 
and  the  great  development  of  the  two  anterior  pygidial  pleura 
into  hollow  spines  directed  outward  and  backward.  These 
features  are  simulated  in  Deiphon,  in  which,  however,  the 


156  STUDIES  IN  EVOLUTION 

prominent  glabella  is  without  distinct  lobes,  and  the  large 
pleural  extensions  of  the  pygidium  do  not  belong  to  the  ante- 
rior segment.  Its  natural  place  is  at  the  end  of  the  series. 
F.  Cowper  Reed  31  has  shown  (in  his  memoir  on  the  evolu- 
tion of  Cheirurus  and  its  sub-genera,  not  including  the  other 
genera  of  the  family)  that  the  direct  line  from  Cyrtometopus 
passes  through  Cheirurus  to  Crotalocephalus.  The  genera 
Pseudosphcerexochus  and  Ampliion  also  have  relations  with 
these  genera  and  should  be  placed  here.  There  is  next  a 
group  of  forms  with  prominent  globular  glabellee,  leading 
from  Cheirurus  to  Sphcerocoryphe,  and  including  Actinopeltis, 
Youngia,  and  Hemisphcerocoryphe.  Staurocephalus  should 
immediately  follow  these.  Sphcerexochus  seems  to  be  related 
to  Cheirurus  and  Actinopeltis.  Like  them  it  has  two  side 
lobes  at  the  base  of  the  glabella,  and  the  anterior  furrows  are 
obsolescent,  as  in  Actinopeltis  and  Youngia.  Lastly  come 
Onycopyge  and  Deiphon,  with  their  globular  glabellse  with- 
out furrows,  the  spiniform  fixed-cheeks,  the  thoracic  and 
pygidial  pleura,  and  the  free-cheeks  reduced  to  almost  noth- 
ing, forming  a  small  part  of  the  doublure  of  the  cephalon. 
The  former  genus  has  four  spiniform  pygidial  pleura,  two  on 
each  side,  but  in  the  latter  two  are  reduced  and  the  remain- 
ing pair  is  greatly  enlarged. 

Family  XIV.     PHACOPIDJE  Salter. 

Glabella  tumid,  widest  in  front.  Free-cheeks  continuous, 
united  anteriorly.  Suture  extending  from  in  front  of  the  genal 
angles  inward  to  the  eyes,  and  then  forward  around  the  glabella. 
Eyes  generally  large,  and  always  with  distinct  facets,  schizo- 
cliroal.  Thorax  with  eleven  segments,  with  grooved  pleura. 
Pygidium  usually  large  and  of  many  segments;  limb  ribbed; 
margin  entire  or  dentate.  Ordovician  to  Devonian. 

Including  the  genera  and  sub-genera  PteopsEmmrich,  Acaste 
Goldfuss,  Chasmops  McCoy,  Coronura  Hall,  Corycephalus  Hall 
and  Clarke,  Cryphceus  Green,  Dalmanites  Emmrich  (Hausman- 
nia  Hall  and  Clarke),  Homalops  Bemele  and  Dames,  Monorachos 
Schmidt,  Odontocephalus  Conrad,  Pterygometpous  Schmidt,  Sym- 
phoria  Clarke,  and  Trimerocephalus  McCoy. 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    157 

The  last  family  of  trilobites  comprises  forms  which  are 
commonly  believed  to  be  the  most  highly  organized  of  the 
class,  and  certain  it  is  that  a  high  degree  of  organization  is 
manifested.  Some  of  the  characters  may  be  considered  as 
progressive,  while  others  are  larval  or  possessed  chiefly  by 
the  most  primitive  families,  and  are  therefore  to  be  looked 
upon  as  regressive.  Schizochroal  eyes  occur  in  no  other 
family,  and  this  feature  is  apparently  indeterminate.  The 
complete  union  of  the  free-cheeks,  carrying  the  doublure  of 
the  sides  and  front  of  the  cephalon,  can  be  best  homologized 
with  similar  structures  in  some  of  the  lowest  genera,  and  is 
a  retention  of  the  complete  ocular  segment.  The  glabella, 
though  considerably  enlarged  anteriorly,  does  not  attain  the 
degree  of  specialization  shown  in  Lichas  and  Acidaspis. 
Only  Chasmops  and  related  forms  (Monorachos,  Homalops, 
Symphoria,  and  Coronura)  have  separate  or  accessory  lobes. 
The  margin  of  the  cephalon  shows  even  greater  diversity 
than  in  any  other  family.  It  may  be  plain  (Phacops,  Cry- 
phceus),  notched  (Corycephalus),  denticulated  (Odontocepha- 
lus),  or  extended  in  front  as  a  spinose,  spatulate,  or  dentate 
process  (Dalmanites  nasutus  Conrad,  D.  tridens  Hall,  etc.). 
The  pygidium  has  a  range  almost  as  great,  though  in  this 
respect  it  is  equalled  in  the  Lichadidse,  Acidaspidse,  and 
some  of  the  Olenidse.  In  America  the  section  typified  by 
Dalmanites  culminated  during  the  lower  Devonian.  Not 
only  are  the  largest  forms  found  here  (Coronura  diurus 
Green,  C.  myrmecophorus  Green,  D.  tridens  Hall,  etc.),  but 
also  the  most  ornate  and  specialized;  as  Corycephalus,  Odon- 
tocephalus,  and  Coronura. 


References. 

1.  Agassiz,  L.,  1873.  —  Methods  of  Study  in  Natural  History,  eighth 

edition. 

2.  Angelin,     N.   P.,     1854.  —  Palaeontologia    Scandinavica.     Pt.    I. 

Crustacea  formationis  transitionis. 

3.  Barrande,  J.,    1852,   1872.  —  Systeme    Silurien  du  centre  de   la 

Boheme.     Part  I.  1852  ;  supplement,  1872. 


158  STUDIES  IN  EVOLUTION 

4.  Beecher,  C.  E.,  1895.  —  Structure  and  Appendages  of  Trinucleus. 

Amer.  Jour.  Sci.  (3),  vol.  xlix. 

5.   1895.  —  The  Larval  Stages  of  Trilobites.     American  Geologist, 

vol.  xvi. 

6.   1896.  —  On  the  validity  of  the  family  Bohemillidse,  Barrande. 

American  Geologist,  vol.  xviii. 

7.  Bernard,  H.  M.,  1892.  —  The  Apodidse.     A  Morphological  Study. 

Nature  Series. 

8.    1894.  —  The   Systematic   Position  of  the   Trilobites.     Quar. 

Jour.   Geol.  Soc.    London,   vol.  1. 

9.   1895 — Supplementary  notes  on  the  Systematic  Position  of 

the  Trilobites.     Quar.  Jour.  Geol.  Soc.   London,  vol.  li. 

10.    1895.  —  The  Zoological  Position  of  the  Trilobites.     Science 

Progress,  vol.  iv. 

11.  Brongniart,  A.,  1822.  —  Histoire  Naturelle  des  Crustace's  fossiles. 

Trilobites. 

12.  Burmeister,  H.,  1843.  —  Die  Organisation  der  Trilobiten. 

13.  Chapman,  E.  J.,  1889.  —  Some  remarks  on  the  classification  of  the 

Trilobites  as  influenced  by  stratigraphic  relations  ;  with  outlines 
of  a  new  grouping  of  these  forms.  Trans.  Roy.  Soc.  Canada, 
vol.  vii. 

14.  Clarke,  J.  M.,  1892.  — Notes  on  the  Genus  Acidaspis.     Report  of 

the  State  Geologist,  N.  Y.  State  Mus.,  44^  Ann.  Rept. 

15.  Corda,  A.  J.  C.  [and  J.   Hawle],  1847.  — Prodrom  einer  Mono- 

graphic der  bohmischen  Trilobiten.  Abhandl.  bb'hm.  Gesell. 
Wiss.,  Prag,  vol.  v. 

16.  Dalman,   J.   W.,   1826.  —  Om   Palseaderna   eller  de    sa  kallade 

Trilobiterna. 

17.  Emmrich,  H.  F.,  1839.  —De  Trilobitis.     Dissertation. 

18.   1844.  —  Zur  Naturgeschichte  der  Trilobiten. 

19.  Gegenbaur,  C.,  1878.  —  Elements  of  Comparative  Anatomy,  Eng- 

lish edition  (Bell  and  Lankester). 

20.  Goldfuss,  A.,   1843.  —  Systematische  Uebersicht  der  Trilobiten 

und  Beschreibung  einiger  neuen  Arten  derselben.  Neues 
Jahrbuch  fur  Mineralogie,  etc. 

21.  Hyatt,  A.,  1889. — Genesis  of  the  Arietidse.     Mem.  Mus.  Comp. 

ZooL,  vol.  xvi. 

22.  Jackson,   R.    T.,  1890.  —  Phylogeny  of    the   Pelecypoda.      The 

Aviculidae  and  their  Allies.  Mem.  Boston  Soc.  Nat.  Hist., 
vol.  iv. 

23.  Kingsley,  J.  S.,   1894. — The  Classification  of  the   Arthropoda. 

American  Naturalist,  vol.  xxviii. 

24.  Lang,  A.,  1891.  —  Text-book  of  Comparative  Anatomy.     English 

translation  by  H.  M.  and  M.  Bernard. 


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25.  McCoy,  F.,  1849.  —  On  the  classification  of  some  British  fossil 

Crustacea,  with  notices  of  new  forms  in  the  university  collec- 
tion at  Cambridge.     Ann.  Mag.  Nat.  Hist.  (2),  vol.  iv. 

26.  Matthew,   G.   F.,  1896.  —  Faunas  of  the  Paradoxides   Beds  in 

Eastern  North  America.     No.    I.    Trans.  N.    Y.   Acad.    Sci., 
vol.  xv. 

27.  Milne-Edwards,    A.,    1873.  —  Recherches    anatomiques    sur    les 

Limules.     Ann.  Sci.  Nat.,  t.  xvii. 

28.  H.,  1834-40.  —  Histoire  naturelle  des  Crustaces. 

29.  GEhlert,  D.-P.,  1895.—  Sur  les  Trinucleus  de  1'ouest  de  la  France. 

Bull.  Soc.  Geol.  France  (3),  t.  xxiii. 

30.  Quenstedt,  F.  A.,  1837.  —  Beitrage  zur  Kenntniss  der  Trilobiten. 

Archivfur  Naturgesch.,  Bd.  I. 

31.  Reed,   F.   R.   Cowper,   1896.  — Notes  on  the  Evolution  of    the 

Genus  Cheirurus.     Geological  Magazine,  vol.  iii. 

32.  Salter,  J.  W.,  1864.  —  A  Monograph  of  British  Trilobites.     Pt.  I. 

Pal.  Soc.,  London,  vol.  xvi. 

33.  Walcott,  C.  D.,  1881.  —  The  Trilobite ;  New  and  Old  Evidence 

relating  to  its  Organization.     Bull.  Mus.  Comp.  Zool ,  vol.  viii. 

34.  Woodward,  Henry,  1895.  —  Some  Points   in  the   Life-history  of 

the  Crustacea  in  Early  Palaeozoic  Times.     Anniversary  Address 
of  the  President.     Quar.  Jour.  Geol.  Soc.  London,  vol.  li. 

35.  Zittel,  K.  A.,  1881-1885.  —  Handjmch  der  Palseontologie,  Bd.  II. 

36.   1895.  —  Grundziige  der  Palseontologie. 


List  of  Genera. 


Page 

Acaste  Goldfuss.  156 

Acerocare  Angelin.  142 

Acidaspis  Murchison.  151 

Acrocephalites  Wallerius.  142 

Actinopeltis  Cord  a.  155 

^Eglina  Barrande.  146 

Aglaspis  Hall.  132 

Agnoslus  Brongniart.  136 

Agraulus  Corda.  142 

Amphion  Pander.  155 

Ampyx  Dalman.  138 

Anacheirurus  Reed.  155 

Ancyropyge  Clarke.  151 

Aneucanthus  Angelin.  140 

Angelina  Salter.  142 


Page 

Anomocare  Angelin.  142 

Anopolenus  Salter.  142 

Arctinurus  Castelnau.  150 

Areia  Barrande.  153 

Arethusina  Barrande.  148 

Arges  Goldfuss.  150 

Arraphus  Angeliu.  137 

Asaphelina  Bergeron.  142 

Asapliellus  Callaway.  146 

Asapliiscus  Meek.  146 

Asaphus  Brongniart.  146 

Atops  Enimons.  140 

Avalonia  Walcott.  140 

Bailiella  Matthew.  140 

Barrandia  McCoy.  146 


160 


STUDIES  IN  EVOLUTION 


Basilicus  Salter.  146 

Bathynotus  Hall.  140 

Bathyurellus  Billings.  146 

Batliyuriscus  Meek.  146 

Bathyurus  Billings.  146 

Bavaritta  Barrande.  142 

Bergeronia  Matthew.  142 

Boeckia  Brogger.  142 

Bohemilla  Barrande.  132 

Bolbocephalus  Whitfield.  •  146 

Brachyaspis  Salter.  146 

Bracliymetopus  McCoy.  148 

Brongniartia  Salter.  154 
Bronteopsis  W.  Thompson.  146 

Bronteus  Goldfuss.  149 

Bumastus  Murchison.  146 

Burmeisteria  Salter.  154 

Calymmene  Brongniart.  154 

Calymmenella  Bergeron.  154 
Calymmenopsis  Munier-Chal- 

mas  and  Bergeron.  154 

Carausia  Hicks.  140 

Carman  Barrande.  140 

Celmus  Angelin.  148 

Centropleura  Angelin.  142 

Ceratocephala  Warder.  151 
Ceralolichas  Hall  and 

Clarke.  150 

Ceratopyge  Corda.  142 

Ceraurus  Green.  155 

Chariocephalus  Hall.  142 

Chasmops  McCoy.  156 

Cheirurus  Bey  rich.  155 

Conocephalites  Barrande.  140 

Conocoryphe  Corda.  140 

Conolichas  Dames.  150 

Conophrys  Callaway.  133 

Cordania  Clarke.  148 

Coronura  Hall.  156 
Corycephalus  Hall  and 

Clarke.  156 

Corynexochus  Angelin.  142 


Page 

Crepicephalus  Owen.  142 

Cromus  Barrande.  153 

Crotalocephalus  Salter.  155 

Crotalurus  Volborth.  148 

Cryphceus  Green.  156 

Cryptonymus  Eichwald.  146 

Ctenocephalus  Corda.  140 

Ctenopyge  Linnarsson.  142 

Cybele  Loven.  153 

Cyclognathus  Linnarsson.  142 

Cyclopyge  Corda.  146 

Cyphaspis  Burmeister.  148 

Cyphoniscus  Salter.  133 

Cyrtometopus  Angelin.  155 

Dalmanites  Emmrich.  156 

Dechenella  Kayser.  148 

Deiphon  Barrande.  155 
Diaphanometopus  Schmidt.    155 

Dicranogmus  Corda.  150 

Dicranurus  Conrad.  151 

Dictyocephalites  Bergeron.  140 

Dikelocephalus  Owen.  142 

Dindymene  Corda.  153 

Dionide  Barrande.  138 

Dipleura  Green.  154 

Dolichometopus  Angelin.  146 

Dorypyge  Dames.  142 

Dysplanus  Burmeister.  146 

Eccoptocheile  Corda.  155 

Ectillcenus  Salter.  146 

Ellipsoceplialus  Zenker.  142 

Elliptocephala  Emmons.  142 

Encrinurus  Emmrich.  153 

Endymionia  Billings.  138 

Erinnys  Salter.  140 

Eryx  Angelin.  140 

Euloma  Angelin.  142 

Eurycare  Angelin.  142 

Gerasaphes  Clarke.  146 

Griffithides  Portlock.  148 

Goldius  de  Koninck.  149 

Harpes  Goldfuss.  137 


NATURAL   CLASSIFICATION  OF    THE   TRILOBITES    161 


Harpides  Beyrich. 
Harpina  Novak. 
Harttia  Walcott. 
Hausmannia  Hall   and 

Clarke. 

JSemisphcerocoryphe  Reed. 
Holasaphus  Matthew. 
Holmia  Matthew. 
Holocephalina  S alter. 
Holometopus  Angelin. 
Homalonotus  Koenig. 
Homalopecten  IS  alter. 
Homalops   Remele   and 

Dames. 

Homolichas  Schmidt. 
Hoplolichas  Dames. 
Hydrocephalus  Barrande. 
Hydrolenus  Salter. 
Illcenopsis  Salter. 
Illcenurus  Hall. 
Illcenus  Dalman. 
Isocolus  Angelin. 
Isotelus  De  Kay. 
Koenigia  Salter. 
Leiolichas  Schmidt. 
Leptoplastus  Angelin. 
Liclias  Dalman. 
Liostracus  Angelin. 
Loganellus  Devine. 
Lonchodomus  Angelin. 
Mer/alaspides  Brogger. 
Megalaspis  Angelin. 
Menocephalus  Owen. 
Mesonads  Walcott. 
Metopias  Eichwald. 
Micmacca  Matthew. 
Microdiscus  Emmons. 
Monorachos  Schmidt. 
Neseuretus  Hicks. 
NieszkowsJcia  Schmidt. 
Nileus  Dalman. 
Niobe  Angelin. 


Page 

Page 

137 

Octillcenus  Salter. 

146 

137 

Odontocephalus  Con  r  ad  . 

156 

140 

Odontopleura  Emmrich. 

151 

Ogygia  Brongniart. 

146 

156 

Ogygiopsis  Walcott. 

146 

155 

Olenelloides  Peach. 

142 

146 

Olenellus  Hall. 

142 

142 

Olenoides  Meek. 

142 

146 

Olenus  Dalman. 

142 

133 

Oncholiclias  Schmidt. 

150 

154 

Onycopyge  Woodward. 

155 

146 

Oryctoceplialus  Walcott. 

142 

Palceopyge  Salter. 

142 

156 

Panderia  Voiborth. 

146 

150 

Parabolina  Salter. 

142 

150 

Parabolinella  Brogger. 

142 

142 

Paradoxides  Brongniart. 

142 

146 

Peltura  Angelin. 

142 

146 

Phacops  Emmrich. 

156 

146 

Phaetonella  Novak. 

148 

146 

Pharostoma  Cord  a. 

154 

133 

Pliillipsia  Portlock. 

148 

146 

Phillipsinella  Novak. 

146 

154 

Placoparia  Corda. 

153 

150 

Plcesiacomia  Corda. 

154 

142 

Platymetopus  Angelin. 

150 

150 

Platypeltis  Callaway. 

146 

142 

Plutonides  Hicks. 

142 

142 

Pnonopeltis  Corda. 

148 

138 

Proceratopyge  Wallerius. 

142 

146 

Proetus  Steininger. 

148 

146 

Prosopiscus  Salter. 

153 

142 

Protagraulus  Matthew. 

142 

142 

Protolenus  Matthew. 

142 

150 

Protopeltura  Brogger. 

142 

142 

Protypus  Walcott. 

142 

136 

Pseudopliillipsia  Gemmellaro. 

156 

148 

142 

PseudospJicerexochus  Schmidt. 

155 

155 

146 

Psilocephalus  Salter. 

146 

146 

Pterocephalia  Roemer. 

142 

11 

162 


STUDIES  IN  EVOLUTION 


Page 

Pterygometopus  Schmidt.  156 

Ptychaspis  Hall.  142 

Ptychometopus  Schmidt.  154 

Ptycltoparia  Corda.  142 

Ptychopyge  Angelin.  146 

Raphiophorus  Angelin.  138 

Remopleurides  Portlock.  142 

Salteria  Walcott.  140 

Salteria  W.  Thompson.  138 

Sao  Barrande.  142 

Sclimidtella  Tschernyschew.  148 

Schmidtia  Marcou.  142 

Selenopeltis  Corda.  151 

Shumardia  Billings.  133 

Solenopleura  Angelin.  142 

Sphcerexochus  Beyrich.  155 

Sphcerocoryphe  Angelin.  155 

Sphceropthalmus  Angelin.  142 

Staurocephalus  Barrande.  155 


Page 

Stygina  Salter.  146 

/Symphoria  Clarke.  156 

Sympliysurus  Goldfuss.  146 

Telephus  Barrande.  142 

Terataspis  Hall.  150 

Thakops  Conrad.  146 

Toxotis  Wallerius.  140 

Triarthrella  Hall.  142 

Triarthrus  Green.  142 

Trimerocephalus  McCoy.  156 

Trimerus  Green.  154 

Trinudeus  Lhwyd.  138 

Triopus  Barrande.  133 

Trochurus  Beyrich.  150 

Tropidocoryphe  Novak.  148 

Uralichas  Delgado.  150 

Xiphogomium  Corda.  148 

Youngia  Lindstrom.  155 

Zacanthoides  Walcott.  142 


2.    THE  SYSTEMATIC   POSITION  OF  THE 
TRILOBITES  * 

As  a  preface  to  these  remarks,  it  may  be  stated  that  there  is 
no  intention  of  indulging  in  a  controversy  regarding  trilobite 
affinities.  Professor  Kingsley,  as  a  biologist  and  authority  on 
living  arthropods,  naturally  approaches  the  subject  from  a 
standpoint  nearly  opposite  to  that  of  a  trilobite  investigator 
or  paleontologist.  The  differences  of  opinion  or  interpreta- 
tion held  by  each  are  generally  more  apparent  than  real,  and, 
as  stated,  depend  mainly  upon  the  point  of  view.  Further, 
it  cannot  be  expected  that  students  of  Lang,  Glaus,  and 
Lankester  will  agree  as  to  the  value  and  significance  of  a 
number  of  important  characters,  or  upon  certain  theories 
which  have  been  the  natural  outcome  of  such  differences. 

In  the  study  of  trilobite  morphology  and  classification  I 
have  made  homologies  and  correlations  from  theories,  opin- 
ions, and  observations  which  seemed  most  current  and  in 
general  favor  in  standard  text-books.  The  chief  purpose  of 
the  investigation  was  to  work  out  the  structure  and  develop- 
ment of  the  trilobite,  and  to  apply  the  information  to  a  classi- 
fication of  the  members  of  the  group  itself.  The  results  have 
been  recently  published  in  the  American  Journal  of  Science 
(February  and  March,  1897).  No  attempt  was  made  to  revise 
the  classification  of  the  animal  kingdom  from  the  trilobite 
standpoint,  nor  even  to  determine  the  branches  of  arthropod 
phylogeny.  The  discussion  of  the  systematic  position  of 
Limulus  was  carefully  avoided,  though  this  is  usually  consid- 
ered the  chief  end  of  any  trilobite  theorizing.  The  affinities 

*  This  paper  was  written  to  follow  one  by  J.  H.  Kingsley,  on  "  The  Sys- 
tematic Position  of  the  Trilobites,"  published  in  the  American  Geologist,  XX,  38- 
40, 1897. 


164  STUDIES  IN  EVOLUTION 

of  the  trilobites  were  manifestly  closer  to  the  Entomostraca 
and  Malacostraca  than  to  other  arthropods,  and  therefore  com- 
parisons were  drawn  with  these  sub-classes  of  the  Crustacea. 

In  the  following  remarks  only  the  main  points  of  difference 
between  the  views  held  by  Professor  Kingsley  and  myself  are 
dwelt  upon :  — 

If  the  trilobites  are  true  crustaceans,  as  conceded,  it  is  then 
fair  to  expect  a  more  or  less  close  agreement  between  the  lar- 
val forms  of  both.  In  my  paper  on  "  The  Larval  Stages  of 
Trilobites"  (American  G-eologist,  September,  1895)  I  endeav- 
ored to  show  this  close  agreement,  and  concluded  that  the 
protaspis  stage  of  trilobites  could  be  homologized  with  the 
nauplius  larva  of  higher  Crustacea.  Professor  Kingsley  notes 
the  following  differences :  (1)  The  differentiated  median  and 
pleural  regions;  (2)  the  segmented  cephalic  region;  (3)  the 
absence  of  a  median  eye :  and  (4)  paired  eyes. 

As  to  the  first,  I  do  not  think  the  differentiation  is  much 
greater  than  in  the  nauplii  of  Apus,  Cyclops,  Lucifer,  and 
others  in  which  there  are  side  regions.  The  pleural  regions 
cannot  be  considered  as  highly  specialized  characters,  since 
they  are  common  to  many  groups,  and  each  segment  is  con- 
sidered as  primarily  consisting  of  tergum,  pleura,  and  sternum. 

(2  )  The  segmentation  of  the  protaspis  is  very  feeble  in  the 
earliest  stages,  and  is  evidently  emphasized  from  the  fact  that 
the  fossils  are  viewed  as  opaque  objects  and  exhibit  strongly 
any  inequalities  of  surface  features,  while  living  nauplii  are 
studied  as  translucent  objects.  Furthermore,  any  such  dif- 
ference cannot  be  real,  since  the  nauplius  shows  its  true  seg- 
mented nature  in  its  paired  appendages. 

(3)  The  apparent  absence  of  a  median  eye  in  the  trilobite 
protaspis  could  be  taken  as  of  some  value  were  it  not  that 
the  fossils  are  not  more  than  one  millimetre  in  length,  and 
even  under  the  most  favorable  conditions  could  hardly  be 
expected  to  show  such  small  features  as  ocelli.  Moreover, 
the  median  eye  may  have  been  marginal  or  ventral,  and  there- 
fore would  not  be  seen  in  the  fossil,  which  preserves  only  the 
dorsal  crust. 


SYSTEMATIC  POSITION  OF  THE   TRILOBITES     165 

(4)  Paired  eyes  are  not  present,  or  at  least  not  visible  in 
the  protaspis  stages  of  primitive  trilobites.  They  may  through 
acceleration  appear  in  the  protaspis  stages  of  later  genera,  as 
they  do  in  the  nauplius  embryos  of  certain  modern  decapods. 

I  do  not  believe  that  the  nauplius  has  any  great  phyloge- 
netic  significance,  and  have  considered  it  "  as  a  derived  larva 
modified  by  adaptation  "  (I.  c.,  p.  190),  and  as  a  "  modified  crus- 
tacean larva  "  (ibid.,  p.  191). 

It  does  not  seem  necessary  to  correlate  the  post-oral  second 
pair  of  trilobite  appendages  with  the  mandibles  of  higher 
Crustacea.  The  second  pair  in  the  nauplius  is  also  post-oral 
and  manducatory,  though  they  later  develop  into  the  antenna 
and  are  pie-oral. 

As  to  the  cephalon  of  a  primitive  crustacean,  I  have  merely 
accepted  the  conclusion  approved  by  Glaus,  as  stated  by  Lang, 
in  his  reconstruction  of  the  original  crustacean,  which  is  as 
follows :  "  The  head  segment  was  fused  with  the  four  subse- 
quent trunk  segments  to  form  a  cephalic  region"  (Comparative 
Anatomy,  p.  406). 

Similarly  in  regard  to  the  interpretation  of  the  biramous 
appendages,  I  have  adopted  the  statements  and  conclusions 
of  a  large  number  of  zoologists  who  consider  the  most  primi- 
tive appendages  as  branched  or  consisting  of  a  dorsal  and  a 
ventral  member,  and  I  have  followed  them  in  thus  interpret- 
ing the  trilobite  appendages,  which  are  clearly  of  this  nature. 


3.    THE   LARVAL   STAGES   OF  TRILOBITES  * 

(PLATES  III-V) 

INTRODUCTION. 

IT  is  now  generally  known  that  the  youngest  stages  of 
trilobites  found  as  fossils  are  minute  ovate  or  discoid 
bodies,  not  more  than  one  millimetre  in  length,  in  which 
the  head  portion  greatly  predominates.  Altogether  they  pre- 
sent very  little  likeness  to  the  adult  form,  to  which,  however, 
they  are  traceable  through  a  longer  or  shorter  series  of  modi- 
fications. 

Since  Barrande  2  first  demonstrated  the  metamorphoses  of 
trilobites,  in  1849,  similar  observations  have  been  made  upon 
a  number  of  different  genera  by  Ford,22  Walcott,34'  35>  36  Mat- 
thew,26' 27>  28  Salter,32  Callaway,13  and  the  writer  4'  5' 7.  The 
general  facts  in  the  ontogeny  have  thus  become  well  estab- 
lished, and  the  main  features  of  the  larval  form  are  fairly 
well  understood. 

Before  the  recognition  of  the  progressive  transformation 
undergone  by  trilobites  in  their  development,  it  was  the 
custom  to  apply  a  name  to  each  variation  in  the  number  of 
thoracic  segments  and  in  other  features  of  the  test.  The 
most  notable  example  of  this  is  seen  in  the  trilobite  now 
commonly  known  as  Sao  hirsuta  Barrande.  It  was  shown 
by  Barrande 3  that  Corda 17  had  given  no  less  than  ten 
generic  and  eighteen  specific  names  to  different  stages  in  the 
growth  of  this  species  alone. 

The  changes  taking  place  in  the  growth  of  an  individual 

*  American  Geologist,   XVI,  166-197,  pis.  viii-x,  1895. 


LARVAL  STAGES  OF  TRILOBITES  167 

are  chiefly :  the  elongation  of  the  body  through  the  gradual 
addition  of  the  free  thoracic  segments  ;  the  translation  of 
the  eyes,  when  present;  the  modifications  in  the  axis  of  the 
glabella  ;  the  growth  of  the  free-cheeks ;  and  the  final  assump- 
tion of  the  mature  specific  characters  of  pygidium  and 
ornamentation. 

In  the  present  paper  the  larval  stages  of  several  species  are 
described  and  illustrated  for  the  first  time,  and  a  review  is 
undertaken  of  all  the  known  early  larval  stages  thus  far 
described.  This  work  would  have  no  special  interest  in 
itself  were  it  not  for  the  fact  that,  with  our  present  under- 
standing of  trilobite  morphology,  it  is  possible  to  reach 
some  conclusions  of  general  importance  which  have  a  direct 
bearing  on  the  significance  and  interpretation  of  several  of 
the  leading  features  of  the  trilobite  carapace,  and  incidentally 
upon  the  structure  and  relations  of  the  nauplius  of  the 
higher  Crustacea. 

The  Protcfspis. 

Barrande  3  recognized  four  orders  of  development  in  the 
trilobites,  as  follows  :  — 

TYPES 
(  Head  predominating,  incomplete.  \ 

I.  •<  Thorax  nothing  or  rudimentary.    >-  Sao  hirsuta. 
(  Pygidium  nothing.  ) 

(  Head  distinct,  incomplete.  ) 

II.  }  Thorax  nothing.  [  Trmculeus  ornatus  and 

(  Pygidium  distinct,  incomplete.  )  a11  Agnostus. 

r  Head  complete.  \ 

III.  -!  Thorax  distinct,  incomplete.  >  Arethusina  Konineki. 

(.  Pygidium  distinct,  incomplete.  ) 

f  Head  complete.  ^ 

IV.  •<  Thorax  complete.  >•  Dalmanites  Hausmanni. 

(  Pygidium  distinct,  incomplete.  ) 

A  study  of  these  groups  shows  at  once  that  they  form 
a  progressive  series  in  which  the  first  alone  is  primitive. 


168  STUDIES  IN  EVOLUTION 

The  others  are  more  advanced  stages  of  development,  as 
shown  by  the  larger  size  of  the  individuals,  and  their  hav- 
ing characters  which  appear  successively  in  the  ontogeny 
of  a  species  belonging  to  the  first  order  of  development. 
To  attain  the  stage  which  is  represented  by  actual  speci- 
mens, they  must  have  passed  through  earlier  stages,  which 
as  yet  have  not  been  found.  Furthermore,  it  is  evident 
that  Barrande  did  not  consider  the  orders  after  the  first  as 
primitive,  and  characteristic  of  the  genera  cited,  for,  in 
some  remarks  under  the  third  order,  he  says:3  "II  est  tres- 
vraisemblable,  que  la  plupart  des  Trilobites  de  cette  sec- 
tion, si  ce  n'est  tous,  devront  etre  un  jour  transfe're's  dans 
la  premiere,  par  suite  de  la  ddcouverte  probable  d'embryons 
sans  segmens  thoracique." 

The  geological  conditions  necessary  for  the  fossilization 
of  the  minute  larval  forms  of  trilobites  are  such  that  only 
in  comparatively  rare  instances  are  any  of  the  immature 
stages  preserved.  Larval  specimens  are  doubtless  often  over- 
looked or  neglected  by  collectors,  but  generally  the  sedi- 
ments are  too  coarse  for  the  preservation  of  these  small  and 
delicate  organisms.  In  certain  horizons  and  rocks,  how- 
ever, such  remains  are  quite  abundant,  and  complete  onto- 
logical  series  may  be  obtained.  Yet  it  is  not  strange  that 
series  of  equal  completeness  have  not  been  found  in  all 
Paleozoic  horizons. 

The  abbreviated  or  accelerated  development  of  many  of 
the  higher  Crustacea  has  resulted  in  pushing  the  typical  free- 
swimming,  larval  nauplius  so  far  forward  in  the  ontogeny 
that  this  stage  is  either  eliminated  or  passed  through  while 
the  animal  is  still  within  the  egg,  so  that  when  hatched  it  is 
much  advanced.  Although  the  trilobites  show  distinct  evi- 
dence of  accelerated  development  through  the  earlier  inherit- 
ance of  certain  characters  which  will  be  taken  up  later,  yet 
it  is  not  believed  that  the  normal  series  or  periods  of  transfor- 
mation were  to  any  degree  disturbed,  since  both  the  simplest 
and  most  primitive  genera  whose  ontogeny  is  known  and  the 
most  highly  specialized  forms  agree  in  having  a  common 


LARVAL  STAGES  OF  TRILOBITES  169 

early  larval  type.  This  would  be  expected  from  their  great 
antiquity,  their  comparatively  generalized  and  uniform  struc- 
ture, and  from  the  fact  that  no  sessile,  attached,  parasitic, 
land,  or  freshwater  species  are  known.  These  conditions, 
by  introducing  new  elements  into  the  ontogeny,  would  tend 
to  modify  or  abbreviate  it  in  various  ways,  especially  among 
the  higher  genera. 

Before  discussing  any  of  the  various  philosophical  and 
theoretical  problems  involved  in  an  attempt  to  correlate  the 
larval  forms  of  Crustacea,  a  brief  consideration  of  the  known 
facts  relating  to  the  larvse  of  trilobites  will  be  presented. 

Minute  spherical  or  ovoid  fossils  associated  with  trilobites 
have  been  described  as  possible  trilobite  eggs,  by  Barrande 3 
and  Walcott,32  but  nothing  is  known,  of  course,  of  the 
embryonic  stages  of  the  animals  themselves.  The  smallest 
and  most  primitive  organisms  that  have  been  detected,  and 
traced  by  means  of  series  of  specimens  through  successive 
changes  into  adult  trilobites,  are,  as  stated  above,  little 
discoid  or  ovate  bodies  not  mqre  than  one  millimetre  in 
length,  as  shown  on  Plates  III  and  IV.  It  is  fair  to  assume 
that  we  have  here  a  general  exhibition  of  trilobite  larval 
stages,  since  the  ten  species  represented  are  from  various 
geological  horizons  belonging  to  the  Cambrian,  Ordovician, 
and  Silurian  sediments,  with  Devonian  types,  and  showing 
the  simple  as  well  as  the  highly  specialized  forms. 

All  the  facts  in  the  ontogeny  of  trilobites  point  to  one  type 
of  larval  structure.  This  is  even  more  noticeable  than  among 
recent  Crustacea,  in  which  the  nauplius  is  considered  as  the 
characteristic  larval  form.  It  is  desirable  to  give  a  name  to 
this  early  larval  type  apparently  so  characteristic  of  all  trilo- 
bites, and  among  different  genera  varying  only  in  features  of 
secondary  importance.  This  stage  may  therefore  be  called 
the  protaspis  (TT^COTO?  primus,  acrTrls  scutum). 

The  principal  characters  of  the  protaspis  are  the  following: 
Dorsal  shield  minute,  varying  in  observed  species  from  .4  to 
1  mm.  in  length;  circular  or  ovoid  in  form;  axis  distinct, 
more  or  less  strongly  annulated;  head  portion  predominating; 


170  STUDIES  IN  EVOLUTION 

glabella  with  five  annulations ;  abdominal  portion  usually  less 
than  one-third  the  whole  length  of  the  shield,  axis  with  from 
one  to  several  annulations ;  pleural  portion  smooth  or  grooved ; 
eyes  when  present  anterior,  marginal,  or  sub-marginal ;  free- 
cheeks  when  present  very  narrow,  marginal. 

Several  moults  took  place  during  this  stage  before  the 
complete  separation  of  the  pygidium  or  the  introduction  of 
thoracic  segments.  When  such  moults  are  recognized  they 
may  be  considered  as  early,  middle,  and  late  protaspis  stages, 
and  designated  respectively  as  anaprotaspis,  metaprotaspis, 
and  paraprotaspis.  They  introduced  various  changes,  such 
as  the  stronger  annulation  of  the  axis,  the  beginning  of  the 
free-cheeks,  and  the  growth  of  the  pygidial  portion  from  the 
introduction  of  new  appendages  and  segments,  as  indicated  by 
additional  grooves  on  the  axis  and  pleura.  Similar  ecdyses 
occur  during  the  nauplius  stage  of  many  living  Crustacea 
before  a  decided  transformation  is  brought  about.  Certain 
of  these  later  stages  have  received  a  distinctive  name,  and 
are  called  the  metanauplius. 

It  is  believed  that  the  protaspis  is  homologous  with  the 
nauplius  or  metanauplius  of  the  higher  Crustacea.  Most  of 
the  reasons  for  this  belief  will  appear  later  in  the  present 
paper ;  some  which  may  be  stated  now  are  as  follows :  — 

(1)  The  size  of  the  protaspis  does  not  differ  greatly  from 
that  of  many  nauplii,  and  represents  as  large  an  animal  as 
could  be  hatched  from  the  bodies  considered  as  the  eggs  of 
trilobites. 

(2)  Some   of  the   sediments   carefully   examined   by   the 
writer    could   preserve   smaller    larval   trilobites   were   such 
originally  present   and   provided  with   a   chitinous   test,    as 
shown  by  the  abundance  of  minute  ostracodes,  and  the  per- 
fection of  detail  in  these  and  other  fossils. 

(3)  The  protaspis  can  be  shown  to  be  structurally  closely 
related  to  the  nauplius,  and  in  a  more  marked  degree  pos- 
sesses some  characters  required  in  the  theoretical  crustacean 
ancestor. 


LARVAL  STAGES  OF  TRILOBITES  171 

Review  of  Larval  Stages  of  Trilobites. 

Matthew  27>  28  has  carefully  described  several  early  larval 
(protaspis)  stages  of  trilobites  from  the  Cambrian  rocks  t)f 
New  Brunswick,  which  are  very  simple  and  primitive,  and 
will  be  noticed  first. 

Solenopleura  Robbi  Hartt;  Plate  III,  figure  1;  from  the 
Cambrian  of  New  Brunswick;  after  Matthew.27  This  larva 
is  very  minute  and  circular  in  outline;  the  glabella  is  ob- 
scurely annulated  and  extends  to  the  anterior  margin,  where 
it  is  expanded ;  the  neck  ring  is  the  only  one  well  defined ; 
the  abdominal  portion  is  less  than  one-third  the  whole  length, 
and  is  limited  by  a  slight  transverse  furrow;  no  traces  of 
eyes  or  free -cheeks  discernible. 

Liostracus  onangondianus  Hartt;  Plate  III,  figure  2;  from 
the  Cambrian  of  New  Brunswick;  after  Matthew.27  This 
form  is  similar  to  the  preceding,  though  larger,  and  with  the 
glabella  more  rapidly  expanding  in  front.  The  neck  segment 
is  the  only  one  which  is  distinct. 

It  should  be  mentioned  that  most  of  the  larval  specimens 
here  described  and  figured  are  preserved  in  fine  shales  and 
slates,  as  casts  of  the  interior  of  the  dorsal  shield,  so  that 
some  features  are  not  as  emphatic  as  on  the  exterior  of  the 
test.  When  well  preserved,  the  axis  always  shows  the  typical 
five  annulations  on  the  cephalon. 

Ptyclioparia  Linnarssoni  Walcott;  Plate  III,  figures  3  and 
4 ;  from  the  Cambrian  of  New  Brunswick ;  after  Matthew.  ^ 
The  earliest  stage  is  slightly  more  elongate  than  the  pre- 
ceding forms.  The  axis  is  narrow,  expanding  in  front  and 
obscurely  annulated,  five  annulations  belonging  to  the  ceph- 
alon, and  one  to  the  pygidium,  which  is  very  short  and 
separated  from  the  cephalon  by  a  distinct  groove. 

The  second  stage  (figure  4)  is  decidedly  more  elongate; 
the  axis  is  more  distinctly  annulated;  the  occipital  pleura 
defined;  and  the  pygidium  is  larger  and  has  an  additional 
segment. 

Ptyclioparia  Kingi  Meek ;  Plate  III,  figures  5,  6,  and  7 ; 


172  STUDIES  IN  EVOLUTION 

from  the  Cambrian  of  Nevada  and  Utah.  Figure  5  represents 
a  cast  of  the  protaspis,  and  shows  a  defined  occipital  ring, 
with  the  axis  slightly  expanded  and  undefined  in  front ;  py- 
gidium  truncate  behind.  Figure  6,  which  is  referred  to  a 
later  stage  (metaprotaspis)  of  the  same  species,  shows  the 
inception  of  several  characters  that  have  not  as  yet  appeared 
in  the  previous  larvae.  The  axis  is  very  strongly  annulated ; 
the  anterior  lobe  is  nearly  as  long  as  the  four  posterior 
annulations  of  the  cephalon,  and  on  each  side  there  is  a 
furrow  representing  the  eye-line  of  the  adult ;  the  free-cheeks 
are  present  as  narrow  marginal  plates,  including  the  genal 
spines;  the  pygidium  shows  two  segments  separated  by  a 
furrow. 

An  adult  Ptychoparia  Kingi  is  shown  in  figure  7,  and  may 
be  taken  as  representing  the  sum  of  the  changes  passed 
through  in  the  development  of  larvae  like  the  preceding, 
belonging  to  the  genera  Solenopleura^  Liostracus,  and  Ptycho- 
paria. The  introduction  and  growth  of  the  segments  of  the 
thorax  are  perhaps  the  most  marked  changes,  but  other  points 
of  importance  to  be  noted  are:  the  comparatively  smaller 
size  of  the  cephalon  and  its  transverse  form ;  the  limitation 
and  recession  of  the  glabella,  which  is  now  rounded  in 
front,  and  only  extends  about  two-thirds  the  length  of  the 
cephalon;  the  growth  of  the  eyes  and  free-cheeks  at  the 
expense  of  the  fixed-cheeks ;  the  increased  segmentation  of 
the  abdomen,  shown  in  the  axial  and  pleural  grooves  on  the 
pygidium. 

Sao  hirsuta  Barrande ;  Plate  ITT,  figures  8,  9,  10,  and  11 ; 
from  the  Cambrian  of  Bohemia ;  after  Barrande.3  The  speci- 
mens of  this  species  are  preserved  as  casts,  and  several  of  the 
features  are  therefore  somewhat  subdued.  The  earliest  or 
anaprotaspis  stage,  represented  in  figure  8,  is  quite  as  primi- 
tive in  most  respects  as  any  of  the  preceding.  It  is  circular 
in  outline,  the  annulations  of  the  axis  are  distinctly  shown 
only  in  the  neck  segment  and  pygidial  portion,  and  the  eye- 
line  is  present.  In  figure  9  of  the  metaprotaspis  quite  an 
advance  is  seen  in  the  development  of  the  free-cheeks  and 


LARVAL  STAGES   OF  TRILOBITES  173 

the  more  pronounced  annulation  of  the  glabella,  together 
with  pleural  grooves  from  the  neck  segment  and  those  of  the 
pygidium.  The  next  stage  (figure  10)  probably  represents 
the  close  of  the  protaspis  stage  (paraprotaspis)  and  the 
inception  of  the  nepionic  condition,  when  the  cephalon  and 
pygidium  are  distinct  and  before  the  development  of  the  free 
thoracic  segments. 

In  considering  the  changes  necessarily  passed  through  by 
these  larvae  previous  to  attaining  their  adult  characters  (fig- 
ure 11)  the  most  notable,  aside  from  increase  in  size  and 
addition  of  the  sixteen  thoracic  segments,  are :  the  appearance 
and  translation  of  the  eyes  pari  passu  with  the  growth  of  the 
free-cheeks ;  the  growth  of  the  border  in  front  of  the  glabella, 
which  now  narrows  anteriorly,  and  terminates  about  one- 
third  the  length  of  the  cephalon  within  the  margin ;  the  less 
distinct  annulation  of  the  glabella;  and  the  development  of 
the  spines  and  tubercles  ornamenting  the  test. 

Triarthrus  Becki  Green;  Plate  III,  figures  12,  13,  and  14; 
from  the  Ordovician,  Utica  slate,  near  Rome,  New  York.  A 
larval  form  of  this  species  was  figured  by  the  writer6  in  1893. 
At  this  time  the  eye-line  was  confused  with  the  anterior  an- 
nulation of  the  axis,  making  the  cephalon  appear  to  have  six 
instead  of  five  annulations.  A  recent  examination  ofa  large 
number  of  specimens  shows  that  five  is  the  invariable  number, 
as  here  represented.  Two  protaspidian  stages  of  this-  species 
have  been  noticed,  differing  chiefly  in  the  size  of  the  pygid- 
ium. Both  agree  in  showing  a  strongly  annulated  axis,  not 
expanded  in  front  and  terminating  some  distance  within  the 
margin.  From  the  first  annulation  a  slightly  elevated  ridge 
on  each  side  indicates  the  eye-line,  and  extends  to  the  mar- 
ginal eye-lobe.  The  adult  form  (figure  14)  shows,  in  addition 
to  several  characters  noted  in  the  previous  species,  the  nearly 
complete  loss  of  the  two  anterior  annulations  of  the  glabella ; 
the  disappearance  of  the  eye-line;  and  the  development  of 
a  row  of  nodes  along  the  axis,  from  the  neck  segment  to  the 
proximal  segment  of  the  pygidium. 

Acidaspis  tuberculata  Conrad;  Plate  IV,  figures  1,  2,  and 


174  STUDIES  IN  EVOLUTION 

3 ;  from  the  Lower  Helderberg  group,  Albany  county,  New 
York.4  Several  of  these  remarkable  larvae  have  been  found 
perfectly  silicified  in  a  limestone  from  which  they  have  been 
freed  by  etching.  In  general  form  they  resemble  the  second  lar- 
val stage  of  Sao  (Plate  III,  figure  9),  but  the  pygidium  is  shorter 
and  the  glabella  does  not  expand  and  terminate  in  the  ante- 
rior margin.  No  eye-line  is  present,  but  the  eye-lobes  may  be 
seen  a  little  within  the  margin.  The  glabella  has  the  charac- 
teristic number  of  annulations ;  margin  provided  with  a  row 
of  denticles ;  genal  angles  extended  into  spines ;  pygidium 
with  four  spines. 

The  adult  condition  (figure  3)  shows  that  the  eyes  have 
moved  inward  and  backward  to  near  the  neck  segment.  The 
glabella  has  lost  its  annulations  and  is  broken  up  into  a 
median  lobe  with  two  smaller  ones  on  each  side,  while  the 
neck  ring  is  projected  into  a  spine.  The  changes  noted  here 
are  much  more  profound  than  in  any  of  the  preceding  genera, 
since  Acidaspis  is  one  of  the  most  highly  specialized  of  trilo- 
bites  in  its  glabellar  structure  and  elaborate  ornamentation. 
The  protaspis,  too,  partakes  of  this  specialization,  and  although 
the  general  form  of  the  shield  and  the  annulation  of  the  axis 
are  as  primitive  as  in  Triarthrus,  yet  the  characteristic  spi- 
nosity  of  the  genus  appears  even  at  this  early  stage  and  is 
a  marked  instance  of  acceleration  of  development. 

Arges  consanguineus  Clarke ;  Plate  IV,  figure  4  ;  from  the 
Lower  Helderberg  group,  Albany  county,  New  York.  A 
single  larval  form  of  this  type  has  been  found,  and  at  first  was 
provisionally  referred  to  Phaethonides*  The  recent  publica- 
tion by  Clarke14  of  Arges  consanguineus  from  the  same 
horizon  and  a  comparison  of  the  larva  with  the  description 
and  with  considerable  additional  material,  renders  it  now 
possible  to  determine  definitely  the  relations  of  this  interest- 
ing form.  As  the  main  details  of  structure  in  Acidaspis  and 
Arges  are  so  similar,  the  transformations  undergone  by  the 
larva  are  much  alike  in  each  case.  The  young  Arges  likewise 
shows  the  same  acceleration  in  the  development  of  the  spines 
and  surface  ornamentation,  and  the  retention  of  the  primitive 


LARVAL  STAGES   OF  TRILOBITES  175 

features  of  the  glabella.  The  specimen  seen  in  figure  4  rep- 
resents a  late  larval  stage  (paraprotaspis),  as  shown  by  the 
transverse  form  of  the  cephalon  and  the  large  size  of  the 
pygidium. 

Proetus  parvimculus  Hall ;  Plate  IV,  figures  5,  6,  and  7 ; 
Utica  slate,  near  Rome,  New  York.  Two  larval  stages  of 
this  species  have  been  found.  The  younger  (figure  5)  is 
smooth,  broadly  ovate,  .72  mm.  long,  and  widest  in  front ; 
axis  distinctly  annulated,  cylindrical  on  the  cephalon,  tapering 
on  the  pygidium ;  eyes  nearly  transverse  to  the  axis,  very 
large  and  prominent,  situated  on  the  anterior  margin,  sepa- 
rated only  by  the  axis.  The  specimen  represented  in  figure  6 
is  in  the  paraprotaspis  stage,  and  measures  .96  mm.  in  length. 
It  shows  an  advance  over  the  other  in  its  size,  its  larger  p}7gid- 
ium  with  grooved  pleura,  and  the  beginning  of  the  recession 
of  the  eyes. 

The  adult  of  this  small  species  is  shown  in  outline  enlarged 
two  diameters,  in  figure  7.  The  principal  changes  from  the 
larva  which  should  be  noticed  are :  the  loss  of  the  four  ante- 
rior annulations  of  the  glabella,  the  neck  segment  being  the 
only  one  wholly  defined,  although  the  basal  lobes  represent 
remnants  of  the  next  anterior ;  the  translation  of  the  eyes 
backward  as  far  as  the  pleura  of  the  neck  segment,  and  the 
change  from  a  transverse  to  a  parallel  position  with  respect 
to  the  axis. 

In  the  original  description  of  this  species,23  no  mention  was 
made  of  fine  undulating  strise  ornamenting  the  entire  dorsal 
surface  of  the  test,  nor  of  the  basal  lobes  of  the  glabella. 
Both  these  features  are  present  in  the  type  specimen,  which  is 
from  Cincinnati,  Ohio,  as  well  as  in  all  the  specimens  from 
the  Utica  slate,  near  Rome,  New  York.  With  these  additional 
characters  the  species  is  very  closely  related  to  Proetus  deco- 
rus  Barrande. 

Dalmanites  socialis  Barrande ;  Plate  IV,  figures  8—11 ;  from 
the  Ordovician  of  Bohemia;  after  Barrande.3  A  nearly  complete 
series  of  the  growth  stages  of  this  species  is  given  by  Barrande. 
The  earliest,  or  anaprotaspis,  stage  found  (figure  8)  exhibits  an 


176  STUDIES  IN  EVOLUTION 

outline  and  axis  similar  to  Acidaspis.  The  eyes  are  quite 
large  and  situated,  as  in  the  same  stage  of  Proetus,  transverse 
to  the  axis,  on  the  anterior  border.  Genal  angles  present,  but 
in  this  case  not  produced  by  the  free-cheeks  as  in  Sao  and 
Ptychoparia  ;  glabella  strongly  annulated,  increasing  in  diam- 
eter anteriorly,  although  not  expanding  at  the  frontal  mar- 
gin as  in  Sao,  etc.  In  the  two  following  stages  (figures  9, 10), 
the  pygidium  increases  in  size,  and  the  pleura  are  defined. 
To  reach  maturity  (figure  11),  eleven  segments  are  devel- 
oped in  the  thorax,  the  glabella  becomes  more  prominently 
developed  in  front,  but  the  five  annulations  are  maintained. 
The  eyes  have  travelled  in  and  back  as  far  as  the  third  cepha- 
lic segment,  and  their  longer  axes  have  swung  around  into  a 
position  parallel  with  the  axial  line,  as  in  Proetus.  The  py- 
gidium has  added  many  new  segments,  and  the  extremity  is 
prolonged  into  a  spine. 

Before  proceeding  further  in  the  discussion  of  the  protaspis, 
it  is  necessary  to  notice  a  number  of  forms  of  young  trilobites 
which  have  heretofore  been  referred  to  the  embryonic  and 
larval  stages,  but  which  are  now  believed  to  belong  to  stages 
later  than  the  protaspis. 

Besides  the  truly  elementary  forms  described  by  Barrande 
and  already  noticed  (Sao  hirsuta  and  Dalmanites  socialis), 
there  are  others  which  he  referred  to  his  second,  third,  and 
fourth  orders  of  development.3  Among  these  Agnostus  may 
be  taken  first.  The  youngest  forms  of  Agnostus  nudus  and 
A.  rex  (figures  76,  77)  measure  respectively  2  and  1.3  mm. 
in  length,  and  the  adults  13  and  15  mm.  The  earliest  stages 
of  the  genera  shown  on  Plates  III  and  IV  measure  less  than 
1  mm.,  while  the  adults  are  more  than  25  mm.,  with  the 
exception  of  Proetus  parviusculus,  which  is  seldom  more  than 
10  mm.  long,  though  this  species  has  a  protaspis  .72  mm.  in 
length.  The  cephalon  and  pygidium  of  the  youngest  known 
Agnostus  are  quite  separate. and  distinct,  which  is  not  the 
case  with  the  typical  protaspis  stage.  It  therefore  seems 
probable  that  on  account  of  the  comparatively  large  size  and 


LARVAL  STAGES  OF  TRILOBITES 


177 


advanced  structure  of  the  youngest  stages  observed,  the  ele- 
mentary forms  of  this  genus  are  as  yet  unknown,  and  possibly 
the  extreme  tenuity  of  the  test  in  the  protaspis  has  prevented 
their  preservation.  In  the  same  way  the  young  of  Trinu- 
cleus  (figure  78)  show  a  separate  cephalon  and  pygidium,  and 
the  specimens  are  in  a  much  more  advanced  stage  of  develop- 
ment than  the  protaspis  of  Proiitm,  shown  on  Plate  IV, 
figure  5.  An  evidence  of  age  is  furnished,  also,  in  the  trans- 
verse shape  of  the  head,  which,  in  typical  elementary  forms, 
is  longer  than  wide  instead  of  wider  than  long. 


76          77 


8 


78 


79 


81 


83 


FIGURE  76.  —  Aynostus  nudus  Beyrich.     (After  Barrande.) 

FIGURE  77.  —  Agnostus  rex  Barrande.     (After  Barrande.) 

FIGURE  78. —  Trinucleus  ornatus  Sternberg.     (After  Barrande.) 

FIGURE  79.  —  Hydrocephalus  saturnoides  Barrande.    (After  Barrande.) 

FIGURE  80. — Hydrocephalus  carens  Barrande.     (After  Barrande.) 

FIGURE  81. —  Olenellus   (Mesonacis)   asaphoides   Emmons  ;    Ford    collection. 

(Original  X  30.) 

FIGURE  82.  —  Olenellus  (Mesonacis)  asaphoides  Emmons.     (After  Ford.) 
FIGURE  83. —  Olenellus  (Mesonacis)  asaphoides  Emmons.     (After  Walcott.) 

The  youngest  specimens  of  Arethusina  KoninM,  figured 
by  Barrande,3  are  2  mm.  or  upward  in  length,  and  have 
seven  or  more  free  thoracic  segments,  with  the  cephalon 
wider  than  long.  The  facts  of  ontogeny  show  that  younger 
stages  must  be  admitted  in  which  the  number  of  segments 
diminishes  to  nothing,  continuing  down  to  a  form  agreeing 
with  the  protaspis  of  other  genera. 

12 


ITS  STUDIES  IN  EVOLUTION 

It  has  already  been  suggested4  that  the  species  described 
by  Barrande  3  under  the  generic  name  of  Hydrocephalm  are 
probably  the  young  of  Paradoxides.  This  conclusion  receives 
further  support  from  the  undoubted  young  of  Olendlus,  a 
related  genus,  which  in  its  immature  stages  bears  a  strong 
resemblance  to  Hydrocephalus.  The  youngest  examples  of 
the  latter  have  a  distinct  pygidium,  a  well-developed  cepha- 
lon,  and  large  eye-lobes  at  the  sides  of  the  glabella,  as  in 
adult  forms.  Free-cheeks  were  evidently  present,  though  not 
generally  preserved.  See  figures  79  and  80. 

The  young  of  Olenellus  asaphoides,  described  and  illus- 
trated by  Ford22  and  Walcott,35'  36  also  present  a  number  of 
features  considerably  in  advance  of  a  typical  protaspis.  The 
immature  characters  are  mainly  the  large  size  of  the  cephalon 
and  the  distinct  annulation  of  the  axis.  The  post-protas- 
pidian  characters  are  the  distinct  and  separate  pygidium,  the 
adult  position  of  the  eyes,  and  the  apparently  well-developed 
free-cheeks.  In  figure  82,  after  Ford,22  the  outer  pair  of 
spines  belongs  to  the  free-cheeks,  the  other  pair  being  formed 
by  the  pleural  extensions  of  the  glabella,  which  were  called 
the  interocular  spines.  See  also  figures  81  and  83. 

The  young  specimen  of  PtycJioparia  monile  Salter  sp., 
figured  and  noticed  by  Callaway,13  is  1.5  mm.  in  length,  and 
agrees,  as  far  as  can  be  determined  without  seeing  the  origi- 
nal, with  what  is  known  of  other  species  of  the  same  genus. 
It  probably  belongs  to  a  stage  later  than  the  protaspis. 

Matthew26  has  carefully  described  some  small  cephala  of 
Ctenocephaliis  (Hartella)  Matthewi  and  Oonocoryphe  (^Baili- 
dld)  Baileyi,  from  the  Cambrian  of  New  Brunswick.  The 
fact  of  their  being  separate  cephala,  transverse  in  form,  and 
from  2  to  3  mm.  in  length,  is  sufficient  to  show  that  they  do 
not  represent  the  youngest  stages  of  these  species. 

The  immature  examples  of  Agnostus,  Trinucleus,  Aretku- 
sina,  Paradoxides^  Olenellus^  Ctenocephalus,  and  Conocoryphe, 
here  briefly  noticed,  are  of  great  interest  in  a  study  of  the 
ontogeny  of  the  various  species  to  which  they  pertain.  In 
the  present  paper,  however,  it  is  intended  chiefly  to  establish 


LARVAL  STAGES  OF  TRILOBITES  179 

the  primary  larval  characters  of  the  trilobites,  and  therefore 
only  the  earliest  stages  are  considered.  Under  the  genera 
just  mentioned  the  writer  has  endeavored  to  show  that  as 
yet  their  ontogeny  cannot  be  traced  as  far  back  as  the  stage 
which  has  been  defined  as  the  protaspis.  Therefore  any 
general  notions  of  first  larval  forms  must  at  present  be  based 
on  the  genera  Solenopleura,  Liostracus,  Ptychoparia,  Sao, 
Triarthrus,  Acidaspis,  Proetus,  and  Dalmanites. 


Analysis  of  Variations  in  Trilobite  Larvce. 

After  taking  a  general  survey  of  the  earliest  known  larval 
stages  of  trilobites  figured  on  Plates  III  and  IV,  it  is  evident 
that  an  accurate  and  detailed  description  of  any  one  would 
not  apply  to  any  other  except  in  certain  broad  characters. 
To  formulate  a  definition  of  the  protaspis  applicable  to  all, 
as  has  been  done  previously  (pp.  169  and  170),  it  is  necessary 
to  neglect  or  eliminate  some  rather  striking  characters  which 
should  now  be  mentioned.  A  few, features  thus  omitted  are 
considered  as  very  primitive  larval  characters,  while  others 
are  modifications  introduced  in  higher  or  later  genera  through 
the  operation  of  the  law  of  earlier  inheritance. 

From  the  best  evidence  now  obtainable,  the  eyes  have 
migrated  from  the  ventral  side,  first  forward  to  the  margin 
and  then  backward  over  the  cephalon  to  their  adult  position, 
thus  agreeing  with  Bernard's  conclusions.12  Therefore  the 
most  primitive  larvae  should  present  no  evidence  of  eyes  on 
the  dorsal  shield,  and  naturally  there  would  be  no  free- 
cheeks  visible.  Just  such  conditions  are  satisfied  in  the 
youngest  larva  of  Ptychoparia,  Solenopleura,  and  Liostracus, 
which  are  the  most  primitive  genera  whose  protaspis  is 
known.  The  eye-line  is  present  in  the  later  larval  and 
adolescent  stages  of  these  genera,  and  persists  to  the  adult 
condition.  In  Sao  it  has  been  pushed  forward  to  the  earliest 
protaspis,  and  is  also  found  in  the  two  known  larval  stages 
of  Triarthrus.  Sao  retains  the  eye -line  throughout  life,  but 
in  Triarthrus  the  adult  has  no  traces  of  it,  and  none  of  the 


180  STUDIES  IN  EVOLUTION 

higher  and  later  genera  studied  has  an  eye-line  at  any  stage 
of  development.  Matthew  has  considered  this  feature  as 
especially  characteristic  of  most  of  the  Cambrian  genera,  and 
now  it  is  further  shown  to  be  a  character  first  appearing  in 
the  later  larval  stages  of  certain  genera  (Ptyclioparia,  etc. ), 
next  in  the  larval  stages  ($20),  then  disappearing  from  adult 
stages  (Triartlirus),  and  finally  pushed  out  of  the  ontogeny 
altogether  (Acidaspis,  Dalmanites,  etc.).  The  eyes  are  visi- 
ble on  the  margin  of  the  dorsal  shield  after  the  paraprotaspis 
stage,  later  than  the  eye-line  in  Ptychoparia,  Solenopleura, 
Liostracus,  Sao,  and  Triarthrus ;  but  in  the  other  genera 
through  acceleration  they  are  present  in  all  the  protaspis 
stages,  and  persist  to  the  mature,  or  ephebic,  condition, 
moving  in  from  the  margin  to  near  the  sides  of  the 
glabella. 

The  changes  in  the  glabella  are  equally  important  and 
interesting.  Throughout  the  larval  stages  the  axis  of  the 
cephalon  is  five-segmented  or  annulated,  indicating  the  pres- 
ence of  as  many  paired  appendages  on  the  ventral  side.  In 
its  simplest  and  most  primitive  state  it  expands  in  front, 
joining  and  forming  the  anterior  margin  of  the  cephalon  (larval 
PtycJioparia,  Sao).  During  later  growth  it  becomes  rounded 
in  front  and  terminates  within  the  margin.  In  higher  genera 
through  acceleration  it  is  rounded  and  well-defined  in  front 
even  in  the  earliest  larval  stages,  and  often  ends  within  the 
margin  (larval  Triarthrus,  Acidaspis).  From  these  common 
types  of  simple,  pentamerous  glabellaB,  all  the  diverse  forms 
among  adult  individuals  of  various  genera  have  been  derived, 
through  changes  affecting  any  or  all  of  the  lobes.  The 
modifications  usually  take  place  in  the  anterior  lobes  first, 
and  gradually  involve  the  others,  though  rarely  disturbing 
the  neck  segment  which  is  the  most  persistent  of  all.  Six 
lobes  are  occasionally  found  in  the  glabella3  of  some  species. 
They  do  not  indicate  an  additional  pair  of  limbs,  for  the 
extra  lobe  is  produced  (a)  by  division  of  the  anterior  lobe 
through  the  greater  or  less  extent  of  the  eye-line  across  the 
axis,  as  in  Olenellus,  Paradoxides  and  Ogygia ;  or  (6)  by  the 


LARVAL  STAGES   OF  TRILOBITES  181 

marked  development  of  muscular  fulcra,  which  are  supposed 
to  be  connected  with  the  hypostoma. 

The  next  structures  not  especially  noticeable  in  all  stages 
of  the  protaspis  are  the  free-cheeks,  which  usually  manifest 
themselves  in  the  meta-  or  paraprotaspis  stages,  though  some- 
times even  later.  Since  they  bear  the  visual  areas  of  the 
eyes,  their  appearance  on  the  dorsal  shield  is  practically 
simultaneous  with  these  organs;  and  before  the  eyes  have 
travelled  over  the  margin  the  free-cheeks  must  be  wholly 
ventral  in  position.  They  are  very  narrow  when  first  dis- 
cernible (Plate  III,  figures  6,  9,  and  10),  and  in  Ptychoparia, 
Sao,  etc.,  include  the  genal  angles,  but  in  Dalmanites  they 
extend  only  a  short  distance  below  the  eyes. 

The  remaining  features  of  the  protaspis  which  here  require 
notice  are  the  pleural  furrows  and  the  pygidium.  The  pleura 
from  the  anterior  segments  of  the  glabella  are  occasionally 
shown,  as  in  the  young  of  Olenellus  (figure  81),  but  usually 
the  pleura  of  the  neck  segment  are  the  first  and  only  ones  to 
be  distinguished  on  the  cephalon,  ,fche  others  being  so  inti- 
mately coalesced  as  to  lose  all  traces  of  their  individuality. 
This  makes  the  cranidium,  or  head  shield,  exclusive  of  the 
free-cheeks,  consist  of  the  fused  lateral  extensions  or  pleura 
of  the  head  segments,  as  already  noticed  by  Bernard.12  The 
possible  pleural  or  segmental  nature  of  the  free-cheeks  will 
be  noticed  later. 

The  distinct  pleura  of  the  pygidium  appear  soon  after  the 
anaprotaspis  stage,  and  in  some  genera  (Sao,  Dalmanites')  are 
even  more  marked  than  in  the  adult  state,  much  resembling 
separate  segments.  The  growth  of  the  pygidium  is  very 
considerable  through  the  protaspis  stages.  At  first  it  is  less 
than  one -third  the  length  of  the  dorsal  shield,  but  by  the 
successive  addition  of  segments,  it  soon  becomes  nearly  one- 
half  as  long.  In  some  genera  it  is  completed  before  the 
appearance  of  the  free  thoracic  segments,  though  usually  new 
segments  are  added  during  the  adolescence  of  the  animal. 

A  number  of  genera  present  adult  characters,  which  agree 
closely  with  some  of  the  larval  features  noticed  in  this 


182  STUDIES  IN  EVOLUTION 

section,  and  are  important  in  a  phylogenetic  study  of  the 
trilobites.  The  main  features  of  the  cephalon  in  the  simple 
protaspis  forms  of  Solenopleura,  Liostracus,  and  Ptychoparia 
are  retained  to  maturity  in  such  genera  as  Carausia  and 
Acontheus,  which  have  the  glabella  expanded  in  front,  join- 
ing and  forming  the  anterior  margin.  They  are  also  without 
eyes  or  eye-line.  Otenocephalus  retains  the  archaic  glabella 
nearly  to  maturity,  and  likewise  shows  eye-lines  and  the 
beginnings  of  the  free-cheeks  (larval  Sao).  Conocoryphe  and 
Ptychoparia  are  still  further  advanced  in  having  the  glabella 
rounded  in  front,  and  terminated  within  the  margin  (larva  of 
Triarthrus).  These  facts  and  others  of  a  similar  nature  show 
that  there  are  characters  appearing  in  the  adults  of  later  and 
higher  genera,  which  successively  make  their  appearance  in 
the  protaspis  stage,  sometimes  to  the  exclusion  or  modifica- 
tion of  structures  present  in  the  most  primitive  larvae.  Thus 
the  larvae  of  Dalmanites  or  Proetus,  with  their  prominent 
eyes,  and  glabella  distinctly  terminated  and  rounded  in  front, 
have  characters  which  do  not  appear  in  the  larval  stages  of 
ancient  genera,  but  which  may  appear  in  their  adult  stages. 
Evidently  such  modifications  have  been  acquired  by  the 
action  of  the  law  of  earlier  inheritance  or  tachygenesis. 
Altogether  it  seems  that  we  have  represented  on  Plates  III 
and  IV  a  progressive  series  of  first  larval  stages  in  exact 
correlation  with  adult  forms,  the  latter  also  constituting  a 
progressive  series,  structurally  and  geologically. 

A  summary  of  the  features  added  to  the  dorsal  shield  of 
the  anaprotaspis  stage  of  acceleration  during  the  evolution 
of  the  class,  from  the  simpler  forms  of  Cambrian  times  to  the 
later  and  more  highly  differentiated  Dalmanites,  Proetus,  and 
Acidaspis,  would  include:  the  free-cheeks;  the  eyes;  the 
more  strongly  lobed  glabella,  rounded  in  front ;  the  transient 
eye-line ;  the  genal  angles ;  and  the  ornaments  of  the  test. 

These  additions,  as  may  be  seen  by  reference  to  Plates  III 
and  IV,  considerably  complicate  and  modify  the  primitive 
protaspis,  but,  as  previously  mentioned,  it  does  not  lose  any 
of  its  essential  structures.  Besides,  it  is  possible  to  trace 


LARVAL  STAGES  OF  TRILOBITES  183 

the  origin  and  significance  of  the  acquired  characters,  and 
thus  to  assign  to  each  its  true  value. 

Antiquity  of  the  Trilobites. 

The  superlative  age  of  the  trilobites  has  been  generally 
recognized,  and  is  too  well  known  to  require  more  than  a 
passing  notice.  Even  in  the  earliest  Cambrian  they  bear 
evidence  of  great  antiquity  in  their  diversified  form,  their 
larval  modifications,  and  their  polymerous  cephalon  and  caudal 
shield,  all  of  which  features  show  that  trilobite  phylogeny 
must  reach  far  back  into  pre-Cambrian  times. 

Not  only  are  the  smallest  species  (Agnostus)  found  in  the 
Cambrian,  but  also  many  of  the  largest  (Paradoxides). 
There  is  a  great  range  of  variation  in  the  number  of  free 
thoracic  segments,  varying  from  two  in  Agnostus  to  twenty 
in  Paradoxides.  The  pygidium  likewise  shows  extreme  vari- 
ation of  from  two  to  upward  of  ten  ankylosed  segments. 
The  eyes  may  be  absent  as  in  Agnostus  and  Microdiscus,  or 
very  large  as  in  Paradoxides,  though  both  in  this  respect 
and  in  the  number  of  somites,  free  or  fused,  the  Cambrian 
genera  are  exceeded  in  later  deposits.  In  ornamentation 
and  spiniform  processes  the  Cambrian  species  show  consider- 
able development,  though  not  as  great  as  others  since  that 
time.  However,  the  wide  variation  they  do  present  in  this 
particular  indicates  differentiation  and  specialization  consider- 
ably removed  from  the  beginning  of  the  trilobite  phylum. 

The  acquisition  of  distinct  larval  stages  could  only  have 
been  reached  through  a  long  series  of  changes  in  ancestral 
forms.  The  composition  of  the  cephalon  and  caudal  shield 
indicates  a  derivation  from  some  primitive  form,  probably 
annelidan,  in  which,  through  adaptation  to  special  require- 
ments, certain  polar  segments  became  fused,  forming  very 
distinct  terminal  body  regions.  Furthermore,  the  trilobites 
are  the  only  large  division  of  the  Arthropoda  which  has  be- 
come extinct.  The  Merostomata  and  Phyllocarida  culmi- 
nated a  little  later,  though  still  represented  by  living  species; 


184 


STUDIES  IN  EVOLUTION 


but  all  the  other  divisions  apparently  have  continued  to 
increase  since  their  inception  during  Paleozoic  time.  The 
only  known  arthropod  contemporaries  of  the  trilobites  in  the 
Cambrian  are  the  Merostomata,  Ostracoda,  Phyllopoda,  and 
Phyllocarida,  all  of  the  higher  forms  apparently  having 
developed  since  that  time.  A  more  graphic  view  of  the 
geological  range  and  distribution  of  the  arthropods  is  repre- 
sented in  the  following  table :  — 

84 


P-i       ^ 


•3       -3 


Cenozoic 

Mesozoic 

Carboniferous 

Devonian 

Silurian 

Ordovician 

Cambrian 

Pre-Cambrian 


FIGURE  84.  —  Geological  Range  and  Distribution  of  Arthropoda. 

Having  thus  far  reviewed  the  features  of  the  primitive  pro- 
taspis  and  some  of  the  characters  it  acquired  through  earlier 
inheritance,  together  with  the  comparative  age  of  the  differ- 
ent groups  of  arthropods,  it  must  be  conceded  that,  in  inter- 
preting crustacean  phylogeny  from  the  facts  of  ontogeny,  the 
trilobites,  so  far  as  they  show  structure,  are  entitled  to  first 
place.  Moreover,  since  the  appendages  are  quite  fully  known 
and  from  them  the  trilobite  proves  to  be  a  most  generalized 
and  primitive  crustacean,  still  greater  reliance  can  be  placed 


LARVAL  STAGES   OF  TRILOBITES  185 

on  deductions  based  upon  a  study  of  this  type.  The  recent 
discoveries  of  the  antennse  and  the  exact  details  of  trilobite 
structure,  together  with  the  larval  homologies  here  made  and 
the  concordance  of  trilobites  with  the  theoretical  original 
crustacean,  leave  almost  no  doubt  as  to  their  true  crustacean 
affinities.  Woodward,37  from  another  point  of  view,  reaches 
the  same  opinion  by  saying :  "  The  trilobita,  being  certainly 
amongst  the  earliest  forms  of  Crustacea  with  which  we  are 
acquainted,  cannot  be  removed  from  that  class  without 
destroying  its  ancestral  record." 

Restoration  of  the  Protaspis. 

At  first  thought  the  attempt  to  reconstruct  the  ventral  side 
of  the  trilobite  protaspis  may  seem  a  little  hazardous  or  pre- 
mature, but  a  careful  consideration  of  all  the  data  leads  the 
writer  to  undertake  this  with  some  confidence. 

The  genus  Triarthrus  is  taken  for  the  basis  of  this  restora- 
tion, as  it  is  to-day  the  best  known  of  all  the  trilobites,  and 
its  ventral  structure  has  been  ascertained  to  a  degree  of  per- 
fection of  detail  which  compares  favorably  with  many  of  the 
recent  crustaceans.6' 7i  8>  9  The  writer  has  studied  the  structure 
of  many  adult  and  immature  specimens,  some  of  them  not  more 
than  5  mm.  in  length,  so  that  fortunately  the  appendages  are 
known  at  many  stages  of  growth.  Especially  are  the  young 
and  rudimentary  limbs  near  the  extremity  of  the  pygidium  in 
adolescent  individuals  of  considerable  morphological  inter- 
est, for  they  agree  closely  with  the  phyllopodiform  trunk 
appendages  in  the  metanauplius  of  Apus,  and  protozoea  of 
Euphausia,  or,  in  a  general  way,  with  the  still  more  rudi- 
mentary trunk  limbs  in  the  nauplius  stages  of  these  and  other 
forms. 

It  has  been  definitely  ascertained  that  the  cephalon  in  trilo- 
bites bears  five  pairs  of  jointed  appendages  or  limbs.9  In  lar- 
val or  immature  specimens,  and  in  adults  in  which  the  glabella 
retains  its  primitive  structure,  this  number  is  indicated  on  the 
dorsal  shield  by  the  five  lobes  or  annulations  of  the  glabella, 


186  STUDIES  IN  EVOLUTION 

including  the  neck  ring.  These  may  therefore  be  taken  as 
representing,  in  so  far,  the  original  segmentation  of  the  cepha- 
lon,  and  agree  with  what  is  generally  accepted  as  the  primitive 
structure  in  modern  true  Crustacea.  The  head  portion  of  the 
protaspis  clearly  shows  this  pentasomitic  structure,  and  evi- 
dently carried  a  corresponding  number  of  paired  limbs  on  the 
ventral  side.  It  has  also  been  demonstrated  that  the  annula- 
tions  on  the  axis  of  the  pygidium  correspond  to  the  number 
of  paired  limbs  beneath,  exclusive,  of  course,  of  the  anal  seg- 
ment. Here,  too,  it  is  possible  to  tell  from  the  pygidial  por- 
tion of  the  protaspis  the  number  of  limbs  present  during  life. 
The  protaspis  of  Triarthrus,  represented  in  Plate  III,  figure 
13,  on  this  basis  had  five  pairs  of  limbs  attached  to  the  head 
portion,  and  two  pairs  to  the  pygidium. 

Next,  as  to  the  composition  and  form  of  these  elementary 
protaspis  limbs,  it  is  safe  to  assume  that  the  anterior  pair, 
corresponding  to  the  antennules,  must  be  uniramous,  since 
they  are  so  during  all  the  young  and  adult  stages  observed, 
and  since  this  form  is  common  to  all  nauplius  stages  of  modern 
Crustacea,  and  is  recognized  as  primitive  and  elementary  for 
the  class.  There  is  apparently  a  greater  similarity  in  the 
larval  antennules  than  between  any  other  appendages,  and  as 
Apus  and  Euphausia  have  these  in  a  very  generalized  form, 
they  are  taken  as  types  of  the  first  pair  of  limbs  of  the  trilo- 
bite  protaspis,  as  shown  in  Plate  V,  figure  1  (J).  It  should 
be  noted,  too,  that  the  antennules  of  the  trilobites  arise  from 
the  sides  of  the  upper  lip  or  hypos toma,  as  in  the  nauplius. 

The  other  head  appendages  are  typically  branched,  though 
in  many  of  the  recent  Crustacea  they  lose  this  character  after 
the  larval  stages.  Especially  is  this  true  of  the  third  pair  of 
limbs,  which  become  modified  into  the  mandibles.  In  trilo- 
bites the  primitive  biramous  structure  of  the  head  limbs  per- 
sists to  adult  stages,  occurring  also  in  limbs  of  all  the  posterior 
segments  where  they  become  more  and  more  phyllopodiform.8 
In  the  restoration  of  the  protaspis  it  seems  only  necessary  to 
append  this  archaic  type  of  limb  to  each  segment,  agreeing  as 
it  does  in  form  and  structure  with  the  rudimentary  limbs  of 


LARVAL  STAGES  OF  TRILOBITES  187 

older  stages  and  with  the  nauplius  and  metanauplius  stages 
of  Apus. 

It  cannot  be  doubted  that  the  protaspis  had  five  pairs  of 
limbs  on  the  head  portion  and  one  or  more  on  the  pygidium, 
and  although  these  are  the  main  points  necessary  to  prove  the 
argument  in  the  next  section,  on  the  nauplius,  yet  it  seems 
perfectly  warrantable  and  better  for  graphic  purposes  to  attach 
the  required  number  of  elementary  limbs  to  the  ventral  side 
of  the  protaspis,  as  represented  in  Plate  V,  figure  1. 

There  are  other  organs  and  structural  details  occurring  in 
the  nauplius  and  in  adult  trilobites,  which  deserve  recogni- 
tion in  a  restoration  of  the  protaspis  stage.  First  among  these 
is  the  labrum,  or  upper  lip.  Nowhere  is  this  plate  so  well 
developed  and  so  striking  a  ventral  feature  as  among  the  tri- 
lobites. There  can  be  no  hesitation,  therefore,  in  accepting 
this  as  characteristic  of  the  protaspis. 

The  trilobites  and  most  recent  crustaceans  have  a  metas- 
toma,  or  lower  lip.  This  is  already  developed  in  the  nauplius 
stage  of  some  Crustacea,  as  EupJiausia  and  Peneus,  and  prob- 
ably represents  an  early  larval  character.  It  usually  appears 
as  a  median  plate  divided  into  two  small  plates,  or  lappets,  on 
each  side  of  the  median  line,  posterior  to  the  mouth,  and  is 
thus  represented  in  the  restored  protaspis.  As  it  occurs  on  a 
segment  bearing  also  a  pair  of  legs  and  has  no  separate  neu- 
romere,  it  cannot  well  be  considered  as  representing  a  somite. 

An  anal  opening  is  found  in  most  nauplii,  especially  in 
those  of  the  non-parasitic  Crustacea,  and  in  those  in  which 
this  stage  is  normal  and  free-swimming.  The  protaspis,  as 
representing  a  free-swimming  larval  stage  of  trilobites,  there- 
fore probably  possessed  an  anal  opening. 

The  only  character  represented  in  the  restoration  which  is 
accepted  purely  from  analogy  is  the  median  unpaired  eye. 
This  organ  is  almost  universally  present  in  the  nauplius,  and 
is  regarded  as  a  very  primitive  character  wherever  found. 

The  next  and  last  structures  to  be  noticed  are  the  free- 
cheeks  and  the  beginnings  of  the  paired  eyes,  as  shown  in 
Plate  V,  figure  1  (#,  oo).  Their  existence  has  already  been 


188  STUDIES  IN  EVOLUTION 

indicated  in  the  descriptions  and  observations  of  the  protaspis 
and  its  derived  characters,  and  need  not  be  repeated  here. 
Apparently  the  nauplius  presents  nothing  homologous,  unless 
possibly  the  frontal  sensory  organs  of  Apus,  Balanus,  Peneus, 
etc.,  may  be  taken  as  such.  The  paired  eyes  and  frontal 
sensory  organs  are  close  together  and  seem  to  have  some  inti- 
mate connection,  for,  as  the  paired  eyes  develop,  the  latter 
dwindle  and  disappeear.  Likewise  in  the  trilobites  the  free- 
cheeks  bear  the  visual  areas,  and  may  be  almost  wholly  con- 
verted into  eyes  as  in  ^iEglina  (Qydopyge). 

The  greater  or  less  separation  of  the  cerebral  ganglia  in  the 
chaetopods  and  in  some  of  the  lower  Crustacea  leads  to  the 
idea  that  the  free-cheeks  in  trilobites  are  the  pleura  of  an 
occuliferous  head  segment,  which  otherwise  is  lost.  If  the 
hypostoma  is  homologous  with  the  annelid  prostomium,  as 
urged  by  Bernard,11  then  the  free-cheeks  may  be  considered 
as  representing  the  second  procephalic  segment,  which  is  the 
number  required  on  the  supposition  that  each  neuromere  cor- 
responds to  a  somite.  There  is  a  separate  neuromere  to  each 
mesodermic  metamere  posterior  to  the  head,  and  from  anal- 
ogy we  should  expect  that  each  neuromere  in  the  cephalon 
would  represent  an  original  segment,  especially  as  it  can  be 
demonstrated  that  the  head  is  composed  of  consolidated  or 
fused  segments  (Kingsley 24). 

Having  thus  shown  the  probable  ventral  structure  of  the 
protaspis,  we  are  prepared  to  make  some  general  observations 
on  the  larval  type  of  modern  Crustacea  known  as  the  nau- 
plius. Before  doing  this  it  is  well  to  emphasize  again  that 
there  is  very  positive  evidence,  amounting  virtually  to  cer- 
tainty, that  the  protaspis  had  five  pairs  of  limbs  attached  to 
the  cephalic  portion,  behind  which  was  an  abdominal  portion 
containing  the  formative  elements  out  of  which  all  the  pos- 
terior somites  and  appendages  were  developed. 

The  Crustacean  Nauplius. 

The  name  Nauplius  was  first  used  by  O.  F.  Miiller29  to 
designate  a  minute  crustacean  believed  to  represent  an  adult 


LARVAL   STAGES   OF  TRILOBITES  189 

animal.  Afterwards  it  was  found  to  be  a  larval  stage  of 
Cyclops,  but  because  it  agreed  in  structure  with  the  larvae  of 
many  other  Crustacea  the  name  was  retained  for  that  type 
of  larval  form  and  is  now  in  general  use.  Primarily  it  is 
supposed  to  represent  the  first  free-swimming  stage  after  the 
escape  of  the  animal  from  the  egg.  However,  many  species 
are  quite  fully  developed  when  leaving  the  egg,  and  undergo 
comparatively  slight  subsequent  metamorphoses,  and  in  these 
and  other  species  there  may  be  developed  in  the  egg  an  em- 
bryo having  some  of  the  characters  of  the  nauplius.  There- 
fore the  term  is  also  applied  to  all  cases  where  a  certain 
assemblage  of  nauplian  characters  occurs  in  the  development 
of  any  crustacean.  Thus  it  may  be  considered  as  a  stage  of 
development  not  restricted  to  a  definite  period  of  ontogeny. 

The  adult  Apus  possesses  so  many  nauplian  features,  and 
in  its  development  passes  through  such  simple  metamor- 
phoses, that  it  has  been  aptly  considered  by  Bernard  ll  as  a 
nauplius  grown  to  maturity.  Balfour1  also  states  that  the 
chief  point  of  interest  in  the  development  of  Apus  "  is  the  fact 
of  the  primitive  Nauplius  form  becoming  gradually  converted 
without  any  special  metamorphoses  into  the  adult  condi- 
tion. "  *  This  form,  together  with  the  nauplii  of  other  crus- 
taceans and  the  study  of  the  larval  and  adult  characters  of 
the  trilobites,  ought  to  afford  definite  knowledge  of  the  char- 
acters possessed  by  the  ancestral  forms  of  the  Crustacea. 

Before  further  examining  the  nauplius  it  may  be  well  to 
state  the  characters  which,  on  the  grounds  of  comparative 
anatomy  and  phylogeny,  are  believed  to  represent  the  primi- 
tive adult  crustacean.  It  will  be  seen  that  in  many  respects 
the  trilobite  recalls  this  type,  but,  as  already  suggested,  is 
removed  some  distance  from  the  prototype,  although  in  itself 
a  most  primitive  crustacean.  Lang  25  gives  a  very  comprehen- 
sive description  of  the  racial  form,  as  follows :  "  The  original 
Crustacean  was  an  elongated  animal,  consisting  of  numerous 

*  The  adult  Apus  properly  has  five  pairs  of  cephalic  limbs.  A  sixth  pair  of 
appendages  has  been  correlated  as  maxillipedes,  though  from  their  innervation 
they  seem  to  be  metastomic  and  homologous  with  the  chilaria  of  Limulus. 


190  STUDIES  IN  EVOLUTION 

and  tolerably  homonomous  segments.  The  head  segment 
was  fused  with  the  4  subsequent  trunk  segments  to  form  a 
cephalic  region,  and  carried  a  median  frontal  eye,  a  pair  of 
simple  anterior  antennae,  a  second  pair  of  biramose  antennae 
and  3  pairs  of  biramose  oral  limbs,  which  already  served  to 
some  extent  for  taking  food.  From  the  posterior  cephalic 
region  proceeded  an  integumental  fold  which,  as  dorsal 
shield,  covered  a  larger  or  smaller  portion  of  the  trunk.  The 
trunk  segments  were  each  provided  with  one  pair  of  biramose 
limbs.  Besides  the  median  eye  there  were  2  frontal  sensory 
organs.  The  nervous  system  consisted  of  brain,  oesophageal 
commissures  and  segmental  ventral  chord,  with  a  double 
ganglion  for  each  segment  and  pair  of  limbs.  The  heart  was 
a  long  contractile  dorsal  vessel  with  numerous  pairs  of  ostia 
segmentally  arranged.  In  the  racial  form  the  sexes  were 
separate,  the  male  with  a  pair  of  testes,  the  female  with  a 
pair  of  ovaries,  both  with  paired  ducts  emerging  externally 
at  the  bases  of  a  pair  of  trunk  limbs.  The  excretory  func- 
tion was  carried  on  by  at  least  2  pairs  of  glands,  the  anterior 
pair  (antennal  glands)  emerging  at  the  base  of  the  second  pair 
of  antennae,  the  posterior  (shell  glands)  at  the  base  of  the 
second  pair  of  maxillae.  The  mid-gut  possibly  had  segmen- 
tally arranged  diverticula  (hepatic  invaginations). " 

The  characters  ascribed  to  the  typical  nauplius  have  been 
selected  mainly  on  the  principle  of  general  average.  They 
do  not  satisfy  the  theoretical  demands  resulting  from  a  com- 
parative morphological  study,  nor  are  they  consistent  with  the 
accepted  requirements  of  an  ancestral  type  of  the  Crustacea. 
Glaus16  urges  that  the  nauplius  is  a  modified  or  secondary 
larval  form,  and  the  writer  now  hopes  to  further  substantiate 
this  view,  and  partly  to  reconstruct  the  nauplius  from  inter- 
nal evidence  and  from  its  more  primitive  representative,  the 
protaspis  of  the  trilobites. 

The  usual  features  attributed  to  the  nauplius  are:  three 
pairs  of  appendages,  afterwards  forming  two  pairs  of  antennae 
and  the  mandibles ;  the  first  pair  is  uniramous  and  sensory  in 
function;  the  second  and  third  pairs  are  biramous,  swimming 


LARVAL  STAGES  OF  TRILOBITES  191 

appendages;  body  usually  unsegmented;  anteriorly  there  is 
a  single  median  eye,  and  a  large  labrum,  or  upper  lip;  an 
alimentary  canal  bent  anteriorly,  and  ending  in  an  anus  near 
the  posterior  end  of  the  body;  a  dorsal  shield;  the  second 
pair  of  antennae  are  innervated  from  a  sub-cesophageal  gan- 
glion. Frontal  sense  organs  and  a  rudimentary  metastoma 
are  sometimes  present.  The  trunk  and  abdominal  regions 
are  not  generally  differentiated. 

Balfour1  remarks  of  the  nauplius  that:  "In  most  instances 
it  does  not  exactly  conform  to  the  above  type,  and  the  diver- 
gences are  more  considerable  in  the  Phyllopods  than  in  most 
other  groups."  This  variation  is  indeed  quite  marked  among 
nearly  all  the  groups  besides  the  phyllopods,  and  furnishes 
the  facts  for  the  conclusion  that  the  hexapodous  condition  is 
not  primitive. 

On  Plate  V  are  represented  some  of  the  leading  types  of 
nauplius  structure,  taken  chiefly  from  the  excellent  compila- 
tion by  Faxon.20  Bearing  in  mind  the  typical  and  average 
characters  of  this  larva,  some  of  the  variations  will  be  briefly 
reviewed. 

The  nauplius  of  Apus,  represented  in  Plate  V,  figure  2, 
shows  the  rudiments  of  five  trunk  segments,  which  in  a  later 
stage  (figure  3)  develop  phyllopodiform  appendages  belong- 
ing to  the  sixth,  seventh,  and  eighth  pairs  of  limbs.  They 
are  the  anterior  trunk  appendages,  and  appear  at  a  time  when 
the  fourth  cephalic  pair  is  a  mere  rudiment  while  the  fifth 
is  entirely  undeveloped.  The  fourth  and  fifth  pairs  of  head 
appendages  evidently  must  have  some  existence,  though 
undeveloped  in  the  nauplius.  The  physical  conditions  of 
nauplius  life  probably  do  not  require  them,  and  they  there- 
fore remain  for  a  time  quiescent  or  undeveloped. 

In  figures  4,  5,  8,  and  6,  respectively,  of  Branchipus, 
Artemia,  Leptodora,  and  Limnaida,  the  first  pair  of  append- 
ages becomes  progressively  shortened,  until,  in  the  last, 
they  almost  disappear.  Leptodora  (figure  8)  and  Lepidurus 
(figure  7)  also  have  rudimentary  trunk  segments  and  append- 
ages (y).  Figures  9  and  10,  of  Daphnia  and  Moina  (from 


192  STUDIES  IN  EVOLUTION 

summer  eggs),  show  how  rudimentary  the  nauplius  append- 
ages may  become  when  this  stage  is  passed  within  the  egg. 
Even  a  more  marked  reduction  is  exhibited  in  the  embryos 
of  Palcemon  and  Astacus  (figures  25  and  26).  Cyclops  is  a 
very  normal  form,  though  even  here  in  a  second  nauplius 
stage  (figure  12)  a  fourth  pair  of  limbs  is  developed. 

Examples  have  been  cited  showing  the  reduction  and  obso- 
lescence of  the  anterior  antennae,  or  first  pair  of  nauplius 
limbs,  and  some  cases  will  now  be  cited  in  which  the  third 
pair  also  becomes  reduced  and  rudimentary.  Achtheres 
(figure  14)  and  Mysis  (figure  22)  afford  instances  of  this 
variation.  The  former  is  of  additional  interest,  as  showing 
that  the  appendages  from  the  fourth  to  the  eighth  may  be 
developed,  while  the  third  remains  quiescent,  and  that  the 
second  pair,  typically  biramous,  is  here  unbranched.  Simi- 
larly, in  Mysis,  Nebalia  (figure  19),  and  especially  in  Cypris 
(figure  18),  the  nauplius  limbs  are  simple.  The  embryo  of 
Lucifer  (figure  24)  and  a  late  nauplius  stage  of  Euphausia 
(figure  21)  are  also  of  moment  in  showing  the  beginnings 
of  the  metastoma  (mt)  with  the  two  maxillae  and  first 
maxillipedes. 

It  appears  from  the  foregoing  facts  that  enough  has  been 
shown  to  prove  the  marked  variations  in  the  number  and 
state  of  development  of  the  nauplius  appendages,  and  to 
reach  the  conclusion  that  potentially  five  pairs  of  cephalic 
appendages  are  present.  The  two  posterior  pairs  are  the 
ones  usually  not  developed  until  after  some  of  the  trunk 
limbs  appear.  Very  satisfactory  explanations  have  been 
offered  as  to  why  the  first  three  pairs  have  been  selected  by 
the  larva,  although  it  does  not  seem  to  have  been  recognized 
that  the  fourth  and  fifth  have  been  more  or  less  suppressed 
daring  the  evolution  of  the  class.  Lang  ^  accounts  for  the 
three  pairs  of  nauplian  limbs  by  saying  that:  "In  a  young 
larva  which,  like  the  Nauplius,  is  hatched  early  from  the 
egg,  only  a  few  of  the  organs  most  necessary  for  independent 
life  and  independent  acquisition  of  food  can  be  developed. 
The  3  most  anterior  pairs  of  limbs  which  serve  for  swimming 


LARVAL  STAGES  OF  TRILOBITES  198 

may  be  described  as  such  most  necessary  organs.  The  third 
pair  perhaps  belongs  to  this  category,  because  as  mouth  parts, 
generally  provided  with  masticatory  processes,  they  serve  not 
only  with  the  others  for  locomotion,  but  also  for  conducting 
food  to  the  oral  aperture." 

Another  point  in  favor  of  the  original  pentamerous  compo- 
sition of  the  cephalic  portion  of  the  nauplius  or  protonauplius 
is  the  dorsal  shield  which  is  present  in  many  forms,  and  is 
considered  (vide  Bernard  n)  as  a  dorsal  fold  of  the  fifth  seg- 
ment. So  that,  in  reviewing  the  nauplius  structures,  we  find 
here  and  there  evidences  of  the  entire  series  of  head  segments. 

Now,  since  the  protaspis  fulfils  the  requirements  by  hav- 
ing five  well-developed  cephalic  segments,  and  is  besides  the 
oldest  crustacean  larva  known,  it  is  believed  that,  in  so  far, 
at  least,  it  represents  the  primitive  ancestral  larval  form  for 
the  class. 

The  nauplius,  therefore,  is  to  be  considered  as  a  derived 
larva  modified  by  adaptation. 

Other  variations  in  the  characters  of  the  nauplius  occur, 
but  as  they  have  clearly  originated  (a)  from  the  parasitic 
habits  of  the  adult,  (6)  from  embryonic  conditions,  or  (c) 
from  earlier  inheritance,  they  need  not  enter  into  considera- 
tion here.  Such,  for  example,  are  (a)  the  absence  of  an 
intestine  in  Sacculina,  (5)  the  absence  of  the  median  eye  in 
Daphnia  and  Moina^  and  (c)  the  bivalve  shell  in  Cypris. 
The  larval  stages  of  other,  and  especially  later  and  higher 
groups  of  arthropods,  offer  more  considerable  differences  and 
need  not  enter  into  this  discussion,  which  is  aimed  chiefly  to 
establish  the  genetic  relationship  between  the  protaspis  of 
trilobites  and  the  nauplius  of  recent  Crustacea. 


Summary. 

Barrande  first  demonstrated  the  metamorphoses  of  trilo- 
bites in  1849,  and  recognized  four  orders  of  development, 
which  are  now  shown  to  be  stages  of  growth  of  a  single 
larval  form. 

13 


194  STUDIES  IN  EVOLUTION 

A  common  early  larval  form  is  recognized  and  called  the 
protaspis. 

The  protaspis  has  a  dorsal  shield,  a  cephalic  portion  com- 
posed of  five  fused  segments  and  a  pygidial  portion  consisting 
of  the  anal  segment  with  one  or  more  fused  segments. 

The  simplest  protaspis  stage  is  found  in  the  Cambrian 
genera  of  trilobites.  During  later  geological  time  it  acquired 
additional  characters  by  earlier  inheritance  and  became  modi- 
fied, though  retaining  its  pentamerous  glabella  and  small 
abdominal  portion. 

Some  of  these  acquired  characters  of  the  dorsal  shield  are 
the  free-cheeks,  the  eyes,  the  eye-line,  the  genal  angles,  and 
the  ornaments  of  the  test.  The  free-cheeks  and  eyes  moved 
to  the  dorsum  from  the  ventrum. 

The  history  of  the  acquired  characters  is  traced  by  means 
of  comparisons  between  larval  and  adult  trilobites,  through 
Paleozoic  time,  and  a  progressive  series  of  larval  forms  estab- 
lished in  exact  correlation  with  adult  forms,  which  them- 
selves constitute  a  progressive  series,  chronologically  and 
structurally. 

The  antiquity  of  trilobites  is  indicated  by  their  remains  in 
the  oldest  Paleozoic  rocks,  and  especially  by  the  fact  that 
in  the  early  Cambrian  they  are  already  much  specialized  and 
differentiated  in  number  of  genera.  The  age  of  the  trilobite 
or  crustacean  phylum  is  further  shown  from  the  distinct 
larval  stages  of  trilobites  and  their  having  a  cephalon  and 
pygidium  of  consolidated  segments. 

Since  the  trilobites  are  among  the  oldest  and  most  general- 
ized of  Crustacea,  their  ontogeny  is  of  considerable  impor- 
tance in  interpreting  crustacean  phylogeny. 

The  protaspis  in  its  segmentation  shows  that  the  cephalon 
had  five  pairs  of  appendages  as  in  the  adult. 

The  crustacean  nauplius  is  shown  to  be  homologous  with 
the  protaspis  and  to  have  potentially  five  cephalic  segments 
bearing  appendages,  which  should  therefore  be  taken  as  char- 
acteristic of  a  protonauplius. 

The  nauplius   is  a  modified  crustacean  larva.     The  pro- 


LARVAL  STAGES  OF  TRILOBITES  195 

taspis  more  nearly  represents  the  primitive  ancestral  larval 
form  for  the  class,  and  approximates  the  protonauplius. 

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logischen,  anatomischen  und  pakeontologischen  Quellen.    Jenaische 
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from  the  Biological  Laboratory,  Johns  Hopkins  Univ.,  vol.  iv,  No.  7. 

22.  Ford,  S.   W.,   1877.  —  On   Some  Embryonic  Forms  of  Trilobites. 

Amer.  Jour.  Sci.  (3),  vol.  xiii. 

23.  Hall,  James,  1860.  —  New   Species  of  Fossils  from  the   Hudson- 

River   Group   of    Ohio  and  other  Western    States.     Appendix, 
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26.  Matthew,  G.  F.,  1884.  —  Illustrations  of  the  Fauna  of  the  St.  John 

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27.   1887.   Illustrations  of  the  Fauna  of  the  St.  John  Group.  No.  IV. 

—  Part  II.     The  Smaller  Trilobites  with  Eyes  (Ptychoparidse  and 
Ellipsocephalidae).     Trans.  Roy.  Soc.  Canada,  vol.  v,  section  iv. 

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Soc.  Roy.  Malac.  de  Belgique. 

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aquis  Daniae  et  Norvegia  reperit,  etc. 

30.  Miiller,  Fritz,  1864.  —  Fiir  Darwin. 

31.  Packard,  A.  S.,  Jr.,  1883.  —  A  Monograph  of  North  American  Phyl- 

lopod  Crustacea.     Twelfth  Ann.  Rept.  U.  S.  Geol.  and  Geol.  Surv. 

32.  Salter,  J.  W.,  1866.  —  A  Monograph  of  British  Trilobites.    Part  III. 

Pal  Soc.,  London,  vol.  xviii. 

33.  Walcott,  C.  D.,  1877.  —  Notes  upon  the  Eggs  of  Trilobites.     Pub- 

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4.   ON  THE  MODE  OF  OCCURRENCE  AND  THE 

STRUCTURE  AND  DEVELOPMENT  OF 

TRIARTHRUS   BECKI* 

(PLATE  VI) 

THE  presence  of  antennae  and  other  appendages  on  a  trilo- 
bite  from  the  Utica  Slate  was  announced  in  May,  1893, 
by  W.  D.  Matthew,  f  The  specimens  were  discovered  by 
W.  S.  Valiant,  J  near  Rome,  New  York,  where  they  occur 
in  a  fine-grained  carbonaceous  shale.  It  was  apparent  that 
specimens  preserving  organs  so  delicate  as  antennae  ought  to 
show,  in  addition,  other  anatomical  features  which  would  be 
of  great  assistance  in  determining  the  zoological  position  of 
the  Trilobita.  With  this  object  in  view,  and  with  the  assist- 
ance of  Professor  Marsh,  a  collection  was  made  for  the  Yale 
University  Museum.  From  this  material  it  is  hoped  that 
the  remaining  details  in  the  structure  of  this  important  fossil 
may  be  made  out.  The  preliminary  examination  of  the  speci- 
mens shows  a  number  of  new  and  remarkable  structural 
features,  some  of  which  will  be  briefly  noticed  here.  It  was 
also  possible  for  the  writer  to  make  observations  in  the  field, 
which  furnish  interesting  facts  as  to  the  mode  of  occurrence 
and  to  the  habits  of  the  trilobite. 

*  Abstract  of  a  paper  "On  the  Structure  and  Development  of  Trilobites," 
read  before  the  National  Academy  of  Sciences,  November  8,  1893.  American 
Geologist,  XIII,  38-43,  pi.  iii,  1894. 

t  On  Antennae  and  other  Appendages  of  Triarthrus  Beckii.  Read  before 
the  N.  Y.  Academy  of  Sciences,  May,  1893.  Published  in  Amer.  Jour.  Sci.  (3), 
XLVI,  121-125,  August,  1893. 

J  Mr.  Valiant  informs  me  that  he  found  the  first  specimen  showing  antennae 
in  1884,  but  it  was  not  until  1892  that  other  specimens  were  obtained  by  him 
and  M.  Sid.  Mitchell  fully  establishing  the  discovery.  The  specimens  sent  to 
Columbia  College  were  collected  by  W.  S.  Valiant,  of  Rutgers  College. 


198  STUDIES  IN  EVOLUTION 

In  their  present  condition  the  specimens  contain  very  little 
calcite  matter,  and  nearly  the  entire  calcareous  and  chitinous 
portions  of  the  animal  are  represented  by  a  thin  film  of  iron 
pyrite.  To  this  kind  of  fossilization  is  doubtless  due  the 
preservation  of  delicate  organs  and  structures  which  other- 
wise would  have  been  destroyed.  For,  as  is  well  known, 
pyrite  may  replace  such  organic  tissues  as  chitine  or  even 
soft  dermal  structures,  the  change  occurring  by  the  slow 
decomposition  of  these  tissues  in  the  presence  of  iron  sul- 
phate in  solution,  or  from  the  action  of  hydrogen  sulphide  as 
a  result  of  decomposition  in  a  chalybeate  water. 

From  the  mode  of  occurrence  of  the  specimens  it  is  evident 
that  some  physical  change  of  a  rather  sudden  nature  must  be 
inferred  to  explain  the  facts.  This  is  shown  from  the  follow- 
ing considerations:  (1)  Their  restricted  vertical  distribution; 
(2)  nearly  all  specimens  are  complete  and  preserve  their 
appendages;  (3)  they  are  of  all  ages,  from  larval  forms  up 
to  full-grown  individuals;  (4)  the  rock  has  a  characteristic 
structure  and  composition;  and  (5)  the  adjacent  strata  con- 
tain a  rather  sparse  fauna  in  which  the  trilobites  are  generally 
fragmentary,  or  usually  without  appendages. 

It  does  not  require  a  violent  catastrophe  to  account  for 
these  peculiarities,  and,  as  in  the  case  of  the  recent  destruc- 
tion of  the  tile-fish  off  the  eastern  coast  of  the  United  States, 
it  is  possible  that  a  temporary  change  in  the  direction  of  an 
ocean  current,  with  the  consequent  variation  of  temperature, 
would  be  amply  sufficient.  Just  what  occurred  in  the  present 
instance  has  not  been  determined.  Throughout  the  trilobite- 
bearing  rocks  generally,  young  and  larval  forms  are  extremely 
rare,  while,  of  full-grown  examples,  fragments  are  the  rule 
and  entire  specimens  the  exception.  Therefore  it  is  believed 
that  the  remains  commonly  found  represent  sheddings  or 
moults,  and  not  in  each  case  the  death  of  a  separate  indi- 
vidual. In  the  present  material,  however,  the  almost  in- 
variable perfection  of  the  specimens  precludes  this  view. 
Moreover,  the  appendages  are  apparently  in  the  position  held 
in  life,  and  not  such  as  obtain  in  the  cast-off  shells  of  recent 
Crustacea. 


TRIARTHRUS  BECKI  199 

Another  feature  noticed  in  the  field  is  that  the  specimens 
nearly  all  lie  with  the  back  down.  The  same  thing  has  been 
observed  by  other  investigators,  and  has  been  accounted  for 
by  the  assumption  that  in  being  drifted  about  along  the 
bottom  such  a  position  would  be  assumed  from  the  centre  of 
gravity  being  on  the  convex  side.  This  idea  does  not  seem 
tenable,  because,  while  on  their  backs,  the  trilobites  would 
be  most  easily  rocked  by  the  currents  of  water,  and  eventu- 
ally be  turned  over  or  dismembered.  A  further  explanation 
has  been  offered  by  Hicks  and  accepted  by  Walcott,*  to  the 
effect  that  trilobites  probably  lived  with  the  ventral  side 
down,  and  the  accumulation  of  gases  in  the  viscera  during 
decomposition  was  sufficient  to  overturn  the  animal  and  allow 
it  to  be  buried  by  the  deposition  of  sediments  in  the  position 
now  found.  This  theory,  also,  does  not  meet  the  facts  as 
here  observed,  for  in  turning  over  a  dead  and  limp  animal 
provided  with  long  and  slender  antennae,  delicate  jointed 
legs,  and  fringed  appendages,  the  legs  would  be  either  folded 
under  the  carapace  on  one  si4e,  or  displaced  from  their 
natural  position.  But,  as  has  been  already  noticed,  the 
present  material  generally  shows  the  legs  extended  on  both 
sides  of  the  body  and  the  antennae  in  a  very  lifelike  position. 
(Plate  VI,  figures  3-7.) 

It  seems  most  probable  that  trilobites  could  both  swim 
freely  and  crawl  along  the  bottom,  and  that,  on  dying,  they 
coiled  themselves  up  in  the  same  manner  as  the  recent 
isopods.  Then  upon  unrolling  they  would  necessarily  lie 
on  their  backs.  Even  if  they  did  not  coil  up,  any  swimming 
animal  having  a  boat -shaped  form  would  settle  downward 
through  the  water  with  the  concave  side  up. 

The  definite  structure  of  the  legs  of  Triarthrus  is  now 
for  the  first  time  clearly  shown,  and  is  of  much  interest. 
Furthermore,  a  difference  can  be  seen  in  the  appendages  of 
the  pygidium,  thorax,  and  cephalon.  Those  of  the  caudal 

*  The  Trilobite :  new  and  old  evidence  relating  to  its  organization.  Bull 
Mus.  Comp.  ZooL,  VIII,  No.  10,  1881. 


200  STUDIES  IN  EVOLUTION 

region  overlap  each  other,  and  are  furnished  with  very  long 
hairs,  or  setae.  The  appendages  of  the  head  include  the 
antennae  and  the  mouth  parts,  the  latter  consisting  of  the 
mandibles  and  maxillae  bearing  palps  and  setae. 

The  legs  of  the  thorax  have  been  worked  out  in  detail, 
and  are  shown  on  Plate  VI,  figures  8,  9.  No  essential  differ- 
ences have  been  observed  in  the  series  attached  to  the  free 
segments.  Each  segment  bears  a  pair  of  biramous  append- 
ages, originating  at  the  sides  of  the  axis,  as  in  other  trilo- 
bites  (Walcott,  I.  c.).  The  anterior  legs  are  the  longest,  and 
the  others  gradually  become  shorter  towards  the  pygidium. 
Those  which  are  here  taken  for  description  are  the  legs  of 
the  second  and  third  free  thoracic  segments.  The  entire 
length  of  the  legs  has  been  exposed  from  the  dorsal  side,  by 
removing  the  overlying  pleurae  of  the  thorax,  which  con- 
cealed nearly  half  their  length.  Each  limb  consists  of  two 
nearly  equal  members,  one  of  which  was  evidently  used  for 
crawling  and  the  other  for  swimming.  These  two  members 
and  their  joints  may  be  correlated  with  certain  typical  forms 
of  crustacean  legs  among  the  Schizopoda,  Cumacea,  and 
Decapoda,  and  may  be  described  in  the  same  terms.  There- 
fore each  limb  is  composed  of  a  stem,  or  shaft,  with  an  outer 
branch  (exopodite)  and  an  inner  branch  (endopodite).  Plate 
VI,  figure  9,  shows  the  joints  of  the  stem  (6,  7),  the  exopo- 
dite (ex,  1  and  0),  and  the  endopodite  (en,  1-5).  The  pre- 
cise form  of  the  coxal  joint  of  the  stem  (coxopodite)  has  not 
yet  been  clearly  made  out.  It  is  followed  by  a  broad  joint 
about  twice  as  long  as  wide,  which  may  be  referred  to  the 
protopodite. 

The  endopodite  (figure  9,  en)  was  the  member  used  for 
crawling,  as  in  the  Schizopoda.  The  three  proximal  joints 
(5,  4->  &)  are  similar  in  form  to  6,  and  taper  gradually  out- 
ward. The  distal  portion  is  completed  by  two  slender  cylin- 
drical joints  (2,  _Z),  the  latter  bearing  at  its  extremity  short 
setae,  or  bristles,  of  which  three  are  commonly  to  be  seen. 

The  other  member,  the  exopodite  (ex),  lies  over  the  en- 
dopodite. It  apparently  articulates  with  the  protopodite,  but 


TRIARTHRUS  BE  OKI  201 

may  spring  from  what  is  here  referred  to  the  coxopodite,  as 
its  basal  portion  is  very  broad  and  originates  close  to  the 
articulation  of  the  protopodite  with  the  coxal  joint.  The 
proximal  joint  of  the  exopodite  (#)  is  somewhat  arched  and 
tapers  rapidly.  It  extends  to  the  ends  of  the  pleurae,  and  is 
the  longest  joint  of  either  branch.  The  posterior  edge  is 
finely  denticulate,  and  carries  a  row  of  long  setae.  The 
distal  portion  (.Z)  is  multiarticulate,  being  composed  of  ten 
or  more  joints.  In  general  form  it  is  slightly  crescentic, 
with  the  margins  thickened,  the  anterior  one  being  strongly 
crenulated.  Long  setae  extend  posteriorly  from  the  crenu- 
lations  on  the  dorsal  side  of  the  leg,  making  a  conspicuous 
fringe  along  the  distal  half  of  the  exopodite. 

Plate  VI,  figure  7,  represents  a  dorsal  view  of  Triarthrus 
Becki,  showing  the  antennae  and  the  exposed  portions  of  the 
appendages.  The  antennae  and  legs  on  the  right  side  are 
drawn  from  one  specimen,  and  the  legs  on  the  left  side  are  as 
shown  in  another  individual.  The  biramous  character  of  the 
entire  series  of  thoracic  legs  is  fvery  evident,  as  is  also  the 
distinction  between  the  crawling  and  swimming  members. 
Figure  8  shows  the  right  second  and  third  legs  of  the  free 
thoracic  segments.  In  figure  9  the  upper  exopodite  is  repre- 
sented without  setae,  so  as  to  bring  out  the  structure  in 
greater  detail.  On  the  lower  leg  the  setae  are  shown. 

The  antennae  are  about  as  long  as  the  head,  and  are  com- 
posed of  short  conical  joints.  They  usually  occur  in  the 
position  shown  in  figures  5  and  7,  but  occasionally  lie  close 
to  the  margin,  figures  3  and  4,  and  sometimes  curve  back- 
ward over  the  head,  as  in  figure  6. 

It  is  not  necessary  in  this  place  to  describe  in  detail  the 
development  of  Triarthrus  Becki,  but  attention  may  be  called 
to  two  early  larval  forms.  The  youngest  is  shown  on  Plate 
VI,  figure  1,  and  may  be  compared  with  the  first  segmented 
stage,  figure  2,  and  with  the  adult,  figure  7.  At  this  early 
stage  the  animal  is  less  than  one  millimetre  in  length  (.63 
mm.),  and  has  no  distinct  separation  into  parts.  The  divi- 
sion into  a  cephalic  and  a  caudal  region  is  indicated  by  a 


202  STUDIES  IN  EVOLUTION 

transverse  groove,  but  as  yet  the  body  segments  are  undevel- 
oped. After  the  separation  of  the  head  and  pygidium  the 
thoracic  segments  are  introduced  successively  between  the 
head  and  abdomen  until  the  full  number  is  reached,  and 
the  animal  measures  from  10  to  55  millimetres  in  length. 
The  segmented  stages  have  been  described  fully  by  Walcott,  * 
and  an  outline  figure  of  the  stage  with  one  thoracic  segment 
is  given  in  figure  2. 

The  final  conclusions  to  be  reached  from  a  complete  study 
of  the  development  and  structure  of  these  animals  can  as  yet 
ba  only  surmised.  It  is  quite  evident,  however,  that  they 
are  related  to  the  true  Crustacea.  The  Trilobita  are  shown 
to  be  a  primitive  type  in  (1)  their  multiple  segmentation,  (2) 
the  irregular  number  of  thoracic  legs,  and  (3)  the  biramous 
structure  of  the  legs.  They  therefore  present  characters 
common  to  the  Entomostraca  and  Malacostraca. 

*  Trans.  Albany  InsL,  X. 


5.    FURTHER   OBSERVATIONS   ON   THE  VEN- 
TRAL  STRUCTURE   OF  TRIARTHRUS* 

(PLATES  VII  and  VIII) 

IN  previous  papers  on  the  ventral  structure  of  Triarthrus^ 
the  anterior  antennae,  thoracic  legs,  and  appendages  of  the 
pygidium  have  been  described.!  There  yet  remain  for  inves- 
tigation the  appendages  of  the  head  and  additional  details  of 
other  parts  of  the  animal.  These  characters  have  not  been 
easily  obtained  on  account  of  the  labor  of  removing  the  rock 
from  such  delicate  structures.  Moreover,  but  few  specimens 
are  in  the  proper  position  or  condition  to  yield  satisfactory 
results.  The  appendages  of  the  bead  either  suffered  greater 
decomposition  than  those  of  the  thorax,  before  mineraliza- 
tion, or  were  so  tenuous  as  to  be  easily  obliterated,  and  are 
now  seldom  sufficiently  well  preserved  for  study.  Further, 
the  number  and  compact  arrangement  of  such  complicated 
organs,  even  when  present,  make  it  difficult  to  trace  their 
precise  form.  A  similar  difficulty  would  be  experienced 
were  one  to  endeavor  to  describe  the  appendages  of  Apus  by 
examining  the  ventral  side  without  cutting  away  some  of 
the  limbs  or  at  least  unfolding  or  bending  them  around. 

*  American  Geologist,  XV,  91-100,  pis.  iv  and  v,  1895. 

t  W.  D.  Matthew. —  On  Antennae  and  other  appendages  of  Triarthrus 
Beckii.  N.  Y.  Academy  of  Sciences,  May,  1893  ;  Amer.  Jour.  Sci.,  August,  1893. 

C.  D.  Walcott.  —  Note  on  some  Appendages  of  the  Trilobites.  Proc.  Biol. 
Soc.  Washington,  March,  1894. 

C.  E.  Beecher.  —  On  the  Thoracic  Legs  of  Triarthrus.  Amer.  Jour.  Sci., 
December,  1893. 

On  the  mode  of  Occurrence,  and  the  Structure  and  Development  of 

Triarthrus  Becki.  American  Geologist,  January,  1894. 

The  Appendages  of  the  Pygidium  of  Triarthrus.  Amer.  Jour.  Sci., 

April,  1894. 


204  STUDIES  IN  EVOLUTION 

The  features  described  in  the  present  paper  have  been  ob- 
tained by  further  work  on  the  material  secured  for  the  Yale 
Museum  by  Professor  Marsh.  No  detailed  review  of  the 
ventral  anatomy  of  Triarthrus  will  be  given  at  this  time, 
only  such  additional  characters  as  have  been  observed  since 
the  publication  of  the  last  paper  on  this  trilobite.  The 
precise  structure  and  relations  of  the  organs  here  described 
must  also  be  left  subject  to  slight  modifications  required  by 
researches  which  are  still  in  progress.  The  writer  has  care- 
fully prepared  the  specimens  and  made  the  drawings  from 
camera-lucida  outlines.  The  appendages,  however,  are  often 
so  faintly  preserved  or  so  obscure  that  in  order  to  represent 
them  in  a  pen-drawing  it  is  necessary  to  emphasize  their 
limits  and  their  prominence,  and  this  may  sometimes  lead 
to  errors  of  interpretation.  It  seems  almost  unnecessary  to 
state  that  errors  are  not  due  to  any  preconceived  notions  of 
trilobite  anatomy,  since  from  the  beginning  of  these  investi- 
gations it  has  not  been  possible  to  predict  with  safety  the 
exact  form  and  details  of  any  of  the  appendages.  Even 
their  presence  has  been  more  or  less  doubtful  until  revealed 
by  a  fortunate  discovery. 

The  paired  appendages  of  the  cephalon  will  be  taken  up  in 
their  order,  beginning  with  the  most  anterior ;  next  the  newly 
observed  characters  of  thoracic  legs ;  then  the  organs  in  the 
median  line,  —  the  hypostoma,  mouth,  metastoma,  and  anal 
opening. 

Close  observation  of  the  specimens  thus  far  prepared  for 
the  purpose  of  showing  the  under  side  of  the  head  fails  to 
detect  more  than  five  pairs  of  appendages  attached  to  the 
cephalon,  apparently  corresponding  to  the  five  typical  limbs 
of  the  crustacean  head.  Considerable  difficulty  is  experi- 
enced in  determining  from  the  ventral  side  of  the  specimens 
the  posterior  limit  of  the  cephalon.  The  ventral  membrane, 
which  alone  is  usually  visible,  does  not  show  marked  evi- 
dence of  segmentation,  and  the  observer  is  guided  chiefly  by 
the  margin  of  the  cephalon,  the  extremities  of  the  pleura, 
and  obscure  transverse  lines  on  the  axial  membrane.  In  a 


VENTRAL  STRUCTURE   OF  TRIARTHRUS  205 

few  cases  the  evident  sliding  or  displacement  of  the  dorsal 
and  ventral  surfaces  further  complicates  the  attempt  to  refer 
the  appendages  to  definite  divisions  of  the  animal. 

Paired  Uniramose  Appendages. 

Anterior  Antennae,  or  Antennules.  —  These  have  been  de- 
scribed by  Matthew,  Walcott,  and  the  writer  (I.  £.).  Wal- 
cott  showed  their  proximal  extremities  and  their  mode  of 
attachment  at  the  side  of  the  hypostoma.  Little  more  can 
now  be  added  except  that  they  are  evidently  the  first  pair 
of  antennal  organs,  and  correspond  to  the  antennules  of  other 
Crustacea.  The  strong  basal  joint  or  shaft  is  shown  in  Plate 
VIII,  figures  9,  10,  11,  attached  to  the  ventral  side  of  the 
head  at  each  side  of  the  hypostoma,  near  the  middle  of  its 
length.  The  shaft  carries  a  single  flagellum,  and  thus  agrees 
with  the  typical  uniramose  antennule  of  the  nauplius  of  Crus- 
tacea. This  simple  antennule  is  still  present  in  the  Isopoda, 
as  in  Mannuopsis  typica.  The  flagella  curve  forward  and  ap- 
proach, nearly  touching  as  they  cross  the  doublure.  Beyond 
the  limits  of  the  head  they  are  variously  disposed,  though 
usually  extending  forward,  at  first  diverging  for  half  their 
length  and  then  slightly  converging  (Plate  VIII,  figures 
5,  6,  7). 

Paired  Biramous  Appendages. 

The  remaining  paired  appendages  of  the  trilobite  all  seem 
to  be  biramous,  and  agree  closely  in  their  general  features. 
Adjacent  members  of  the  series  present  very  slight  differ- 
ences. It  is  only  when  the  primitive  and  simple  phyllopodous 
legs  of  the  pygidium  are  compared  with  the  anterior  thoracic 
or  cephalic  appendages  that  variations  of  note  can  be  ob- 
served, although  these  are  of  form  and  not  of  structure.  On 
this  account  there  is  no  well-defined  separation  into  pos- 
terior antennae,  mandibles,  maxillae,  maxillipeds,  thoracic, 
and  pleopodal  appendages.  It  is  most  convenient,  therefore, 
to  number  them  from  before  backward,  and  to  indicate 


206  STUDIES  IN  EVOLUTION 

homologous  positions  with  other  Crustacea  only  when  there 
is  some  evident  reason  for  so  doing. 

First  Pair  of  Biramous  Appendages,  or  Posterior  Antennae.  — 
The  second  pair  of  appendages,  corresponding  to  the  posterior 
antennae,  are  attached  to  the  head  at  each  side  of  the  glabella, 
on  a  line  with  the  extremity  of  the  hypostoma.  They  are 
apparently  biramous,  and  thus  agree  with  the  second  pair  of 
nauplian  limbs  and  with  the  typical  posterior  antennae  of  many 
Entomostraca  and  Malacostraca.  They  may  be  compared  with 
the  posterior  antennae  in  EupJiausia  pellucida,  one  of  the  schiz- 
opods,  especially  with  the  Furcilia  and  Cyrtopia  stages.  The 
details  of  the  endopodite  and  exopodite  are  not  clearly  shown. 
The  former  is  more  commonly  preserved,  and  its  distal  joint 
extends  just  beyond  the  edge  of  the  carapace.  The  coxopo- 
dite  is  developed  into  a  triangular  plate,  the  inner  angle 
carrying  a  masticatory  ridge,  the  whole  extending  about 
three-fourths  the  distance  from  the  side  of  the  glabella  to 
the  median  line,  just  below  the  hypostoma,  and  directed 
obliquely  backward  (Plate  VIII,  figures  8-11). 

Second  Pair  of  Biramous  Appendages,  or  Mandibles.  —  The 
appendages  here  correlated  with  the  mandibles  are  immedi- 
ately behind  the  first  pair  of  biramous  limbs.  The  proximal  por- 
tion, or  coxopodite,  is  similar  in  form  to  the  preceding,  though 
somewhat  smaller,  and  overlapping  its  basal  part.  The  palps, 
or  endopodial  and  exopodial  branches,  have  not  been  distinctly 
traced,  though  their  presence  is  indicated  on  Plate  VII,  fig- 
ure 1,  where,  on  the  left  side,  there  are  endopodites  and  exop- 
odites  in  sufficient  number  for  each  appendage  of  the  head. 
That  these  should  be  referred  to  the  cephalic  limbs  is  further 
indicated  by  their  being  in  advance  of  the  endopodite,  which 
manifestly  pertains  to  the  first  thoracic  segment.  The  inner 
edge  of  the  mandibles  as  well  as  that  of  the  other  gnathobases 
of  the  head  is  apparently  finely  denticulate,  as  shown  on  Plate 
VII,  figure  1,  and  Plate  VIII,  figure  2. 

Third  and  Fourth  Biramous  Appendages,  or  Maxillae.  — 
Following  the  appendages  referred  to  the  mandibles  are  two 
pairs  of  strong  limbs,  with  broad  plate-like  basal  portions,  or 


VENTRAL  STRUCTURE   OF  TRIARTHRUS  207 

coxopodites,  serving  as  gnathites  (Plate  VIII,  figures  8-11). 
They  resemble  each  other,  and  are  similar  in  form  to  the  two 
preceding  limbs,  though  somewhat  larger.  They  are  usually 
fairly  well  preserved,  and  their  form  and  structure  can  be 
approximately  made  out.  The  endopodites  are  composed  of 
stout  joints,  and  could  be  extended  but  a  short  distance 
beyond  the  margin  of  the  head.  The  exopodites  are  more 
slender,  and  carry  an  abundance  of  stiff  setae,  which  often 
diverge  in  a  fan-like  manner  from  their  line  of  attachment. 
These  brushes  of  setse  occupying  the  cavities  of  the  cheeks 
are  often  preserved  in  specimens  where  the  other  details  of 
the  limbs  are  obscure  or  obliterated.  In  Triarthrus  they  are 
evidently  homologous  with  similar  brushes  observed  by  Wal- 
cott  in  Calymmene.* 

This  completes  the  number  of  paired  appendages  which  can 
be  definitely  referred  to  the  head.  It  is  evident  they  do  not 
differ  conspicuously  from  each  other,  and,  as  will  be  presently 
shown,  they  closely  resemble  the  thoracic  legs  in  all  essential 
structural  characters.  , 

Thoracic  Legs.  —  In  the  paper  by  the  writer  (I.  <?.)  describing 
the  structure  of  the  thoracic  legs,  the  endopodites  and  exopo- 
dites of  the  second  and  third  pairs  were  illustrated,  together 
with  their  points  of  attachment.  The  form  of  the  coxopoclite, 
or  basal  portion,  was  at  that  time  unknown.  With  the 
present  material  it  is  possible  to  add  several  details.  The 
most  important  are  the  inward  prolongation  of  the  coxopodite 
of  each  limb  toward  the  axial  line,  forming  a  gnathobase,  and 
the  progressive  development  of  this  member.  First  it  has 
a  slender  cylindrical  form  in  the  posterior  half  of  the  series, 
then  becomes  flattened  and  denticulate,  and  finally  widens, 
until  on  the  head  it  forms  the  triangular  plate-like  coxopodite, 
with  masticatory  ridge  and  functioning  as  a  gnathite  (Plate 
VII,  figure  1 ;  Plate  VIII,  figures  1-4,  8-11). 

The  large  basal  portions  of  the  limbs  of  Asaphus,  in  the 
specimen  illustrated  by  Walcott  (Science,  March,  1884),  are 

*  The  Trilobite  :  New  and  old  Evidence  relating  to  its  Organization.  Bull 
Mus.  Cornp.  ZooL,  VIII,  No.  10,  1881. 


208  STUDIES  IN  EVOLUTION 

evidently  the  gnathobases,  as  will  be  seen  at  once  from  a  com- 
parison with  Triarthrus  (Plate  VIII,  figure  1). 

Organs  in  the  Median  Line. 

The  Hypostoma.  —  There  is  nothing  peculiar  in  the  hypos- 
toma  of  Triarthrus,  since  it  presents  features  commonly 
found  in  many  other  genera.  It  is  longer  than  wide,  and 
extends  more  than  half  the  length  of  the  head.  The  posterior 
end  is  narrowly  rounded,  margined  by  a  slight  doublure,  and 
often  presents  a  transverse  elevation  near  the  apex,  as  shown 
on  Plate  VIII,  figure  9.  This  may  represent  a  correspond- 
ing hollow  on  the  inner  side  to  allow  for  movements  of  the 
manducatory  organs. 

In  considering  the  exact  location  of  the  appendages  of  the 
head,  it  must  be  understood  that  in  their  present  positions 
they  are  probably  somewhat  displaced.  During  the  process 
of  fossilization  the  whole  inner  tissues  of  the  animal  were 
removed  without  replacement,  allowing  the  ventral  membrane 
to  come  more  or  less  in  contact  with  the  under  side  of  the 
dorsal  crust,  and  thus  causing  some  stretching  of  the  mem- 
brane and  consequent  displacement  of  the  organs.  The 
hypostoma,  being  more  rigid  and  attached  in  front  to  the 
margin  of  the  head,  doubtless  was  not  shifted,  but  dropped 
down  into  the  cavity  of  the  glabella.  When  raised  to  its 
natural  position,  it  probably  extended  a  little  over  the  mouth 
parts.  The  fact  that  the  mouth  and  lower  lip  do  not  come 
opposite  the  organs  correlated  as  mandibles  may  be  due  in 
part  to  the  unequal  stretching  of  the  membrane  over  the  un- 
even inner  surface  of  the  dorsal  crust.  The  extended  gnatho- 
bases directed  obliquely  backward  and  lying  in  the  axial 
hollow  cause  the  appendages  to  appear  as  though  originating 
further  back  than  is  really  the  case.  Nevertheless  the  pos- 
terior position  of  the  second  and  third  pairs  of  appendages,  or 
the  antennae  and  mandibles,  with  respect  to  the  mouth,  does 
not  offer  any  serious  difficulty.  As  shown  by  Lankester,* 

*  Observations  and  Reflections  on  the  Appendages  and  on  the  Nervous 
System  of  Apus  cancriformis.  Quar.  Jour.  Mic.  Sci.,  XXI,  1881. 


VENTRAL  STRUCTURE   OF  TRIARTHRUS          209 

they  were  doubtless  originally  post-oral  in  the  Crustacea,  as 
is  indicated  from  their  innervation  from  the  ventral  nerve 
ganglion  chain  and  not  from  the  archicerebrum  of  the  prosto- 
mium,  or  cephalic  lobe.  Besides,  in  the  embryo  of  Limulus 
all  the  appendages  are  post-oral,  as  shown  by  Packard.* 

The  Mouth.  — Although  the  opening  of  the  mouth  itself 
has  not  been  observed  in  Triarthrus,  there  can  be  little  doubt 
as  to  its  exact  location,  since  it  must  have  been  situated  in 
the  median  line  between  the  hypostoma  and  metastoma.  As 
both  these  organs  are  quite  close  together,  the  place  of  the 
mouth  would  be  as  indicated  on  Plate  VIII,  figure  11,  m. 
Further  corroborative  evidence  may  be  had  from  the  genus 
Calymmene,  in  which  the  mouth  was  determined  by  Walcott 
(I.  c.)  to  lie  at  the  end  of  the  hypostoma,  opening  obliquely 
backward. 

The  Metastoma.  —  The  lower  lip,  or  metastoma,  here  repre- 
sented for  the  first  time,  is  generally  clearly  shown  as  a 
convex  arcuate  plate  just  posterior  to  the  extremity  of  the 
hypostoma.  On  each  side,  at  the  angles,  are  two  small  ele- 
vations, or  lappets,  which  suggest  similar  structures  in  many 
higher  Crustacea,  and  apparently  represent  the  entire  metas- 
toma in  Apus  and  some  other  forms  (Plate  VIII,  figures  9 
and  11,  met). 

The  Anal  Opening.  —  In  tracing  the  intestinal  canal  as  pre- 
served in  Trinucleus,  Barrande  determined  its  posterior  termi- 
nation to  be  at  the  extremity  of  the  pygidium,  and  Bernard  f 
has  recently  succeeded  in  reaching  a  similar  conclusion,  from 
his  studies  of  Calymmene^  in  which  the  anal  opening  was 
found  just  at  the  inner  margin  of  the  doublure  of  the  pygid- 
ium, in  the  median  line.  Plate  VII,  figure  1,  of  Triarthrus 
shows  the  anus  in  the  same  position,  outlined  by  a  slightly 
elevated  wrinkled  ring. 

*  The  Development  of  Limulus  polyphemus.  Mem.  Boston  Soc.  Nat.  Hist., 
II,  1872. 

t  The  Systematic  Position  of  the  Trilobites.  Quar.  Jour.  Geol.  Soc.  London, 
L,  1894. 

14 


210  STUDIES  IN  EVOLUTION 


Observations. 

With  these  additional  discoveries  relating  to  Triarthrus, 
several  observations  upon  its  general  organization  and  com- 
parisons with  other  Crustacea  may  be  made.  This  cannot  be 
done  exhaustively  or  comprehensively  at  this  time,  and  only 
a  few  points  will  be  touched  upon.  The  simplicity  and 
primitiveness  of  the  trilobite  structure  will  first  impress  the 
student.  The  variable  number  of  segments  in  the  thorax 
and  pygidium  in  the  different  genera  shows  the  unstable 
metameric  condition  of  the  class.  The  head  alone  seems  to 
have  a  permanent  number  of  segments  and  appendages. 
Even  this  is  not  often  apparent,  but  the  constant  number 
of  five  head  segments  in  larval  trilobites  shows  this  to  be  the 
true  number,  although  subsequent  growth  may  obscure  or 
obliterate  this  pentasomitic  character,  as  has  been  shown  by 
the  writer  in  Acidaspis  (Amer.  Jour.  Sci.,  August,  1893) 
and  observed  in  other  genera. 

With  the  exception  of  the  antennules,  all  other  paired 
appendages  of  the  animal  seem  to  agree  in  every  point  of 
structure,  and  vary  only  in  the  relative  development  of  cer- 
tain parts.  The  appendages  of  the  pygidium  are  ontogeneti- 
cally  the  youngest,  and  express  the  typical  phyllopodiform 
structure.  Passing  anteriorly,  the  joints  become  less  leaf- 
like,  until  in  the  anterior  thoracic  legs  they  are  quite  slender, 
and  the  limbs  resemble  those  of  schizopods.  Corresponding 
to  this,  there  is,  through  the  whole  series,  a  gradual  develop- 
ment of  a  process  from  the  coxopodite,  forming  a  gnathobase 
to  the  limb.  On  the  head  these  serve  as  true  manducatory 
organs.  Posteriorly  they  were  like  the  basal  endites  of 
Apus,  and  enabled  the  trilobite  to  convey  food  along  the 
entire  length  of  the  axis  to  the  mouth. 

Bernard  (I.  c.)  has  made  a  strong  exposition  of  the  evi- 
dence in  favor  of  the  phyllopod  affinities  of  the  Trilobita, 
and  especially  of  their  relations  to  Apus.  A  portion  of  the 
under  side  of  the  head  of  Apus  is  introduced  for  comparison 
on  Plate  VIII,  figure  12.  Both  pairs  of  antennal  organs 


VENTRAL   STRUCTURE   OF  TRIARTHRUS  211 

(Z,  #)  are  rather  rudimentary  in  this  genus,  and  are  situated 
further  forward  than  in  Triarthrus.  The  powerful  mandibles 
(3)  are  partly  covered  by  the  labrum,  or  hypostoma  (hy). 
Then  follow  two  well-developed  gnathobases,  representing 
the  maxillae  (-4,  £),  the  more  slender  maxilliped  (£),  and  the 
large  first  thoracic  limbs  (7),  behind  which  are  the  basal 
endites,  or  gnathobases,  of  two  of  the  phyllopodous  append- 
ages (£,  9).  The  general  similarity  of  the  cephalic  organs 
of  Apus  and  Triarthrus  is  quite  apparent.  The  most  con- 
spicuous differences,  as  the  absence  of  normal  endopodial  and 
exopodial  elements,  disappear  in  a  study  of  the  ontogeny  of 
the  limbs  of  Apus,  thus  bringing  these  organs  in  the  two 
groups  into  nearly  exact  correlation. 

There  are,  however,  important  structural  features  of  other 
parts  of  the  body,  which  are  quite  dissimilar  from  Apus  and 
the  higher  Crustacea,  and  the  exact  relations  of  the  trilobite 
with  any  one  group  cannot  be  considered  as  fixed.  Points  of 
likeness  may  be  established  with  almost  every  order,  showing 
chiefly  the  relationship  between  the  trilobite  and  the  ancestors 
of  modern  Crustacea. 

Summary  of  Ventral  Organs  of  Triarthrus. 

A  pair  of  appendages  to  each  potential  segment  of  the 
animal,  all  of  which  are  biramous  except  the  anterior  pair. 

Five  pairs  of  appendages  on  the  cephalon. 

Anterior  antennae,  or  antennules,  attached  at  the  sides  of 
the  hypostoma;  simple,  with  a  single  many-jointed  flagellum. 

First  pair  of  biramous  limbs,  or  posterior  antennae,  with 
endopodite  and  exopodite;  basal  portion  manducatory  in 
function. 

Second  pair  of  biramous  limbs,  or  mandibles,  similar  to 
preceding. 

Third  and  fourth  pairs  of  biramous  limbs,  or  maxillae,  same 
as  preceding,  with  large  gnathobases,  well-developed  endopo- 
dites,  and  fringed  exopodites. 

Thoracic  limbs  biramous;  endopodite  a  jointed  crawling 
leg;  posteriorly  the  joints  become  flattened  and  leaf -like; 


212  STUDIES  IN  EVOLUTION 

exopodite  fringed  with  setse,  and  developed  into  a  swimming 
organ;  coxopodite  with  gnathobase. 

Appendages  of  the  pygidium,  true  phyllopodous  limbs. 

Hypostoma. 

Mouth  between  hypostoma  and  metastoma. 

Metastoma,  a  convex  crescentic  plate,  with  side  lappets. 

Anus  in  median  line,  near  ventral  extremity  of  pygidium. 


6.   THE  MORPHOLOGY  OP  TRIARTHRUS* 

(PLATE  IX) 

MOST  of  the  recent  advances  in  the  knowledge  of  trilobite 
structure  have  come  from  the  study  of  Triarthrus.  Since 
Valiant's  discovery  of  the  antennae,  and  its  announcement  by 
Matthew  in  1893,  the  writer  has  published  a  series  of  papers 
on  the  detailed  structure  of  this  trilobite.  Much  time  has 
also  been  spent  in  carefully  working  out  the  numerous  speci- 
mens from  the  abundant  material  in  the  Yale  Museum. 
Altogether  upward  of  five  hundred  individuals  with  append- 
ages more  or  less  complete  have"  been  investigated,  and  at 
the  present  time  it  may  be  safely  said  that  the  important 
exoskeletal  features  have  been  seen  and  described,  f 

Notwithstanding  the  amount  of  information  regarding  the 
details  of  the  various  organs,  very  little  has  been  shown  illus- 
trating the  general  appearance  of  the  animal  with  the  append- 
ages in  a  natural  and  lifelike  position,  and  it  is  one  object 
of  the  present  article  to  supply  this  deficiency. 

Several  specimens  have  been  lately  developed  which  pre- 
serve not  only  the  appendages  in  great  perfection,  but  also 
show  them  extended  and  disposed  in  a  very  lifelike  manner. 
No  new  structural  points  are  here  brought  out,  yet  the 
representation  of  the  complete  animal  serves  as  a  summary 
of  present  knowledge,  and  also  gives  a  definite  picture  of 

*  Amer.  Jour.  Sci.  (4),  L,  251-256,  pi.  viii,  1896.  Reprinted  in  Geological 
Magazine  (London),  dec.  iv,  III,  193-197,  pi.  ix,  1896. 

t  The  more  important  literature  relating  to  the  structure  of  the  genus 
Triarthrus  is  given  at  the  end  of  the  present  article  ;  numbers  in  the  text  refer 
to  this. 


214  STUDIES  IN  EVOLUTION 

great  assistance  in  forming  a  conception  of  general  trilobite 
morphology. 

The  dorsal  view  represented  on  Plate  IX  is  from  a  camera 
drawing  based  upon  three  specimens  of  about  the  same  size. 
One  gives  the  entire  series  of  legs  down  to  the  ninth  free 
segment,  with  the  exception  of  the  exopodites  of  the  head, 
which  are  supplied  from  a  second  individual.  In  the  third 
specimen  the  anterior  appendages  are  bent  and  irregularly 
arranged,  while  from  the  ninth  backward  to  the  end  of  the 
pygidium  they  are  complete  and  uniformly  extended.  The 
figure  is,  therefore,  a  restoration  only  in  so  far  as  represent- 
ing the  best  portions  of  three  individuals. 

The  ventral  view  (Plate  IX)  is  based  mainly  upon  two 
very  excellent  specimens.  One  was  figured  on  Plate  IV, 
vol.  xv,  of  the  American  Geologist,  and  another,  since  found, 
nearly  completes  the  ventral  aspect.  The  under  side  of  the 
head  and  pygidium  was  carefully  compared  with  all  the 
available  material,  and  no  attempt  was  made  to  supply  any 
characters  except  as  to  the  exact  number  of  joints  in  the 
endopodial  cephalic  elements  and  the  precise  form  of  the 
cephalic  exopodites,  which,  from  every  character  observed, 
and  from  analogy  with  similar  structures  elsewhere,  were  as 
represented. 

So  many  specimens  preserve  the  appendages  in  the  position 
shown  in  the  figures,  that  this  must  be  recognized  as  natural, 
and  one  likely  to  have  been  assumed  by  the  living  animal 
when  extended.  Few,  however,  show  the  details  of  the 
limbs  with  sufficient  clearness  to  enable  one  to  make  out  all 
their  joints  and  more  minute  characters. 

In  comparison  with  what  is  now  known  of  the  appendages 
of  several  other  genera  of  trilobites,  especially  Trinucleus,* 
those  of  Triarthrus  seem  to  have  been  exceptionally  long. 
On  this  point  Bernard,  in  a  letter  to  the  writer,  suggests 
that  "  Triarthrus  must  have  been  a  sort  of  '  Daddy  longlegs  ' 
among  the  Trilobites,  as  Scutigera  is  among  the  Myriapoda. " 

*  Structure  and  Appendages  of  Trinucleus.  Amer.  Jour.  Sci.  (3),  XLIX, 
April,  1895. 


MORPHOLOGY  OF  TRIARTHRUS  215 

The  entire  length  of  a  thoracic  leg,  including  the  coxal  joint, 
is  nearly  equal  to  the  width  of  the  body  at  that  point,  and 
about  half  the  length  projects  beyond  the  pleura. 

The  limbs  of  the  head  diminish  in  length  forward  until 
the  anterior  pair  scarcely  extends  beyond  the  border  of  the 
cephalon.  The  anterior  thoracic  legs  are  the  longest,  and 
there  is  a  gradual  shortening  backward  in  the  series,  espe- 
cially noticeable  after  passing  the  fifth,  those  at  the  extremity 
of  the  pygidium  being  about  one-ninth  the  length  of  the  first 
thoracic  leg.  Their  position  is  also  of  interest.  At  the 
posterior  extremity  they  point  almost  directly  backward, 
while  those  on  the  head  are  directed  more  or  less  forward. 
Between  these  two  extremes  all  the  intermediate  positions 
occur  in  regular  order. 

The  gnathobases,  or  coxopodites,  become  more  and  more 
specialized  anteriorly,  growing  broader  and  having  their  inner 
edge  denticulate,  until  on  the  head  they  function  as  true 
manducatory  organs.  The  second  pair,  however,  correspond- 
ing to  the  mandibles  of  higher  .Crustacea,  has  not  become 
clearly  differentiated  from  the  rest  of  the  series,  and  appar- 
ently has  not  lost  the  exo-  and  endopodial  branches. 

Few  changes  of  importance  can  be  traced  in  the  exopo- 
dites,  though  the  latter  are  considerably  reduced  in  size  on 
the  cephalon.  Over  the  anterior  half  of  the  thorax  they 
functioned  as  vigorous  paddles,  and  on  the  pygidium  their 
length  and  compact  arrangement  made  them  overlap  each 
other,  thus  producing  two  broad  flaps,  or  fin-like  organs. 
The  conclusion  cannot  be  avoided  that  Triarthrus  must  have 
been  an  active  creature,  and  with  its  rows  of  endopodites  and 
exopodites  it  was  as  fully  equipped  as  the  bireme  in  classic 
navigation.  The  form  of  the  animal  and  the  multiplicity  of 
locomotor  organs  were  well  adapted  for  rapid  motion  either 
along  the  sea-bottom  or  through  the  water. 

The  youngest  and  most  immature  limbs  are  on  the  pygid- 
ium, and  in  a  }roung  trilobite  they  are  very  much  like  those 
in  the  larval  Apus 4  and  are  typically  phyllopodiform.  Ac- 
cording to  the  law  of  morphogenesis,  these  limbs  may  be 


216  STUDIES  IN  EVOLUTION 

taken  as  of  phylogenetic  value  and  indicative  of  the  primitive 
type  of  limb  structure. 

The  whole  series  of  endopodites  anterior  to  the  last  two 
or  three  show  modifications  from  the  phyllopodous  type,  the 
change  involving  progressively  from  one  to  all  of  the  endites. 
The  endopodites  of  the  pygidium  have  a  true  phyllopodiform 
structure,  and  are  composed  of  broad  leaf-like  joints,  wider 
than  long.  This  character  is  gradually  lost  in  passing  ante- 
riorly, the  distal  endites  being  the  ones  first  affected.  By 
the  time  the  anterior  pygidial  limb  is  reached,  the  three 
distal  joints  are  longitudinally  cylindrical.  The  ninth  tho- 
racic endopodite  shows  a  fourth  endite  becoming  cylindrical, 
and  on  the  first  and  second  thoracic  legs  even  the  proximal 
ones  are  thus  modified,  making  all  the  endites  of  these  limbs 
slender  in  form. 

This  gradual  modification  of  a  phyllopodiform  swimming 
member  into  a  long,  jointed,  cylindrical,  crawling  leg  de- 
serves more  than  passing  notice,  for  here,  probably,  better 
than  in  any  known  recent  form,  can  the  process  and  its  signifi- 
cance be  studied.  No  living  type  of  crustacean  more  nearly 
conforms  to  the  theoretical  archetype  of  the  class  than  do  the 
trilobites,  and  as  Triarthrus  belongs  to  an  ancient  Cambrian 
family,  it  may  be  expected  to  retain  very  primitive  characters. 

In  this  genus  several  causes  evidently  influenced  the  modi- 
fication of  the  appendages.  First  may  be  mentioned  the 
specialization  into  oral  organs  of  the  gnathobases  of  the  head, 
which  would  tend  toward  a  reduction  of  the  other  portions 
of  the  limbs.  Next,  the  assumption  of  a  walking  habit  would 
gradually  lead  to  a  corresponding  adaptation  of  the  anterior 
thoracic  endopodites,  this  region  of  the  body  being  naturally 
the  place  where  they  would  be  most  operative.  Lastly,  any 
tendency  to  change  the  form  of  the  anterior  limbs  would 
be  accelerated  through  the  greater  number  of  moults  they 
undergo  as  compared  with  the  abdominal  appendages. 

Since  the  anal  segment  of  Crustacea  contains  the  formative 
elements  out  of  which  all  the  trunk  segments  are  successively 
developed,  it  may  be  considered  as  the  same  segment  in  all 


MORPHOLOGY  OF  TRIARTHRUS  217 

Crustacea,  no  matter  how  many  nor  what  kinds  of  segments 
may  intervene  between  it  and  the  head.  The  youngest  seg- 
ment, therefore,  is  always  in  the  budding  zone,  just  in  front 
of  the  telson,  or  terminal  somite,  and  those  further  anterior 
and  more  differentiated  are  older.  This  sequential  order  in 
the  age  of  the  segments  and  appendages  may  be  greatly 
obscured  in  higher  forms,  so  that,  as  in  the  Thoracostraca, 
the  last  pair  of  pleopods,  forming  with  the  telson  the  caudal 
fin,  appears  at  an  early  stage  of  the  ontogeny.  In  such 
cases,  as  Lang  says,  "the  grade  of  development  and  physi- 
ological importance  of  a  section  of  the  body  or  of  a  pair  of 
limbs  in  the  adult  animal  may  be  recognized  by  the  earlier 
or  later  appearance  of  their  rudiments."* 

In  Triarthrus  these  disturbing  factors  are  hardly  to  be 
recognized,  for  no  pair  of  limbs  had  an  excessive  physio- 
logical importance  over  any  other  pair  or  series  of  pairs,  and 
increase  progressed  regularly  by  the  addition  of  new  members 
in  front  of  the  anal  segment.  The  pygidium  being  formed 
of  fused  segments  accommodated  itself  to  this  kind  of  growth 
by  pushing  forward  the  series  of  limbs  and  by  the  formation 
of  a  new  free  segment  at  the  posterior  end  of  the  thorax. 
This  process  of  metameric  growth  continued  from  the  pro- 
taspis  stage  with  no  free  thoracic  segments,  and  successively 
added  segment  after  segment  with  corresponding  moults, 
until  the  full  complement  was  reached,  after  which  the 
moulting  resulted  mainly  in  increase  in  size.  The  repetition 
of  moults  afforded  the  chief  means  by  which  modifications  in 
the  appendages  could  be  brought  about. 

The  earliest  protaspis  stage  shows,  from  the  segmentation 
of  the  axis,  that  there  were  present  five  pairs  of  append- 
ages on  the  head  and  two  on  the  pygidium.6  The  adult 
animal  has  thirteen  or  fourteen  free  thoracic  segments  and 
six  pygidial.  f  Now,  so  far  as  is  known  of  trilobite  ontogeny, 
there  was  never  more  than  one  segment  added  at  a  single 

*  Text-Book  of  Comparative  Anatomy,  English  edition  (Bernard),  p.  410. 
t  A  few  individuals  of  this  species  (T.  Becki)  have  been  observed  with  one  or 
two  additional  thoracic  segments.     Walcott.11 


218  STUDIES  IN  EVOLUTION 

moult,  though  there  is  no  evidence  that  there  may  not  have 
been  more  moults  than  segments  between  the  protaspis  stage 
and  the  finished  segmentation.  In  Triarthrus  the  average 
full  number  of  segments  was  attained  by  the  time  the  animal 
reached  a  length  of  about  7  mm.  So  that  the  limbs  of  the 
anterior  thoracic  segment  in  an  individual  7  mm.  in  length, 
and  containing  the  full  complement  of  fourteen  free  and  six 
pygidial  segments,  must  have  undergone  at  least  seventeen 
moults.  The  second  thoracic  segment,  therefore,  at  this 
stage  of  growth  would  have  been  moulted  sixteen  times,  the 
fifth  thirteen  times,  the  tenth  eight  times,  and  the  fourteenth 
four  times.  The  length  of  full-grown  individuals  is  from 
25  to  40  mm.,  and  to  have  reached  this  size  a  considerable 
number  of  additional  moults  must  have  occurred  in  which  all 
the  segments  participated  alike. 

Some  mention  should  be  made  of  the  probable  method  of 
respiration  of  Triarthrus.  No  traces  of  any  special  organs 
for  this  purpose  have  been  found  in  this  genus,  and  their 
former  existence  is  very  doubtful,  especially  in  view  of  the 
perfection  of  details  preserved  in  various  parts  of  the  animal. 

The  delicacy  of  the  appendages  and  ventral  membrane  of 
trilobites  and  their  rarity  of  preservation  are  sufficient  demon- 
stration that  these  portions  of  the  outer  integument  were  of 
extreme  thinness,  and  therefore  perfectly  capable  of  perform- 
ing the  function  of  respiration.  Similar  conditions  occur  in 
most  of  the  Ostracoda  and  Copepoda,  and  also  in  many  of 
the  Cladocera  and  Cirrepedia,  where  no  special  respiratory 
organs  are  developed. 

The  fringes  on  the  exopodites  in  Triarthrus  and  Trinucleus 
are  made  up  of  narrow,  oblique,  lamellar  elements  becoming 
filiform  at  the  ends.  Thus  they  presented  a  large  surface 
to  the  external  medium,  and  partook  of  the  nature  of  gills. 
But,  as  Gegenbaur  says,  "the  functions  of  respiration  and 
of  locomotion  are  often  so  closely  united  that  it  is  difficult  to 
say  whether  certain  forms  of  these  appendages  should  be 
regarded  as  gills,  or  feet,  or  both  combined."  *  For  purposes 

*  Elements  of  Comparative  Anatomy,,  English  edition  (Bell  and  Lankester), 
p.  241. 


MORPHOLOGY  OF  TRIARTHRUS  219 

of  locomotion,  the  limbs  of  the  cephalon  and  pygidium  were 
of  feeble  assistance  compared  with  those  on  the  thorax,  and 
in  the  higher  Crustacea  these  two  regions  are  the  ones  where 
the  greatest  branchial  specialization  takes  place. 

References. 

1.  Beecher,  C.  E.,  1893.  — A  Larval  Form  of  Triarthrus.    Amer.  Jour. 

Sci.  (3),  vol.  xlvi,  pp.  361,  362,  November. 

2.    1893.  —On  the  Thoracic  Legs  of  Triarthrus.     Amer.  Jour.  Sci. 

(3),  vol.  xlvi,  pp.  467-470,  December.     Abstract  of  a  paper  "  On 
the  Structure  and  Development  of   Trilobites,"  read  before  the 
National  Academy  of  Sciences,  November  8. 

3.   1894.  —  On  the   mode  of  occurrence,  and  the  structure  and 

development  of    Triarthrus  Becki.     American  Geologist,  vol.  xiii, 
pp.  38-43,  pi.  iii,  January.     Abstract  of  a  paper  "  On  the  Struc- 
ture and   Development  of   Trilobites,"  read  before  the  National 
Academy  of  Sciences,  November  8,  1893. 

4.    1894.  — The  Appendages  of  the  Pygidium  of  Triarthrus.    Amer. 

Jour.  Sci.  (3),  vol.  xlvii,  pp.  298-300,  pi.  vii,  April.     Read  before 
the  Connecticut  Academy  of  Arts  and  Sciences,  March  21. 

5.   1895.  —  Further  observations  on  the  ventral  structure  of  Tri- 

t 

arthrus.   American  Geologist,  vol.  xv,  pp.  91-100,  pis.  iv, v,  February. 

6.   1895.  — The  Larval  Stages  of  Trilobites.     American  Geologist, 

vol.  xvi,  pp.  166-197,  pis.  viii-x,  September. 

7.  Bernard,  H.  M.,  1894.  —  The  Systematic  Position  of  the  Trilobites. 

Quar.  Jour.  Geol.  Soc.  London,  vol.  1,  pp.  411-432,  August.  Read 
March  7. 

8.   1895.  —  Supplementary  notes  on  the  Systematic  Position  of  the 

Trilobites.     Quar.  Jour.   Geol.  Soc.  London,  vol.  li,  pp.  352-359, 
August.     Read  April  24. 

9.   1895.  —  The   Zoological  Position  of  the   Trilobites.     Science 

Progress,  vol.  iv,  pp.  33-49,  September. 

10.  Matthew,  W.  D.,  1893.  — On  Antennae  and  other  Appendages  of 

Triarthrus  Beckii.  Amer.  Jour.  Sci.  (3),  vol.  xlvi,  pp.  121-125, 
pi.  i,  August.  Read  before  the  New  York  Academy  of  Sciences, 
May,  and  published  in  Trans.  N.  Y.  Acad.  Sci.,  vol.  xii,  pp.  237- 
241,  pi.  viii,  July  22. 

11.  Walcott,  C.  D.,  1879. —Fossils  of  the  Utica  Slate.     Trans.  Albany 

Inst.,  vol.  x,  pp.  18-38,  pis.  i,  ii,  1883.  Author's  extras  printed  in 
advance,  June,  1879. 

12.    1894.  —  Note  on  some  Appendages  of  the   Trilobites.     Proc. 

Biol.  Soc.   Washington,  vol.  ix,  pp.  89-97,  pi.  i,  March  30.     Read 
March     24.      Geological    Magazine,   N.    S.    dec.   iv,   vol.    i,    pp. 
246-251,  pi.  viii,  June. 


7.    STRUCTURE   AND   APPENDAGES   OF 
TRINUCLEUS  * 

(PLATE   X) 

TRINUCLEUS  departs  so  widely  from  the  common  type  of 
trilobite  form  that  any  contribution  of  new  facts  regarding  its 
structure  and  appendages  is  a  matter  of  interest.  Moreover, 
this  added  information  will  be  of  assistance  in  interpreting 
some  peculiar  and  striking  features  in  the  natural  group  of 
genera  of  which  Trinucleus  is  evidently  a  member. 

For  the  present  it  is  convenient  to  consider  in  this  group 
such  forms  as  Trinucleus,  Harpes,  Harpides,  Dionide,  and 
Ampyx.  Most  of  these  have  the  genal  angles  extending  to 
or  beyond  the  pygidium,  with  a  broad,  finely  perforated  or 
punctate  margin  around  the  head.  They  are  further  char- 
acterized by  the  absence  or  obsolescence  of  visual  organs, 
while  the  facial  sutures  are  either  peripheral,  as  in  Harpes, 
or  in  addition  include  the  genal  spines,  as  in  Trinucleus^ 
Dionide,  and  Ampyx.  Several  other  genera  have  been  recog- 
nized as  having  affinities  with  those  mentioned,  but  they  are 
imperfectly  known,  and  will  be  merely  noticed  here.  Har- 
pina  Nova"k,  based  upon  the  features  of  the  hypostoma,  is 
probably  of  only  sub-generic  value  under  Harpes.  Arraphus 
Angelin  is  apparently  based  upon  a  specimen  of  Harpes 
denuded  of  the  punctate  border.  Salteria  of  W.  Thompson 
and  Endymionia  of  Billings,  both  generally  considered  as 
closely  related  to  Dionide,  were  founded  upon  too  imperfect 
material  to  afford  decisive  data  as  to  their  affinities.  Ange- 
lin's  sub-genera  of  Ampyx  (Lonchodomus,  Raphiophorus,  and 

*  Amer.  Jour.  Set.  (3),   XLIX,  307-311,  pi.  iii,  1895. 


STRUCTURE  AND  APPENDAGES  OF  TRINUCLEUS    221 

Ampyx)  are  based  upon  the  length  of  the  glabellar  spine,  and 
the  possession  of  five  or  six  free  thoracic  segments.  Similar 
characters  in  Trinudeus  are  not  considered  as  worthy  of  such 
marked  distinction. 

In  1847  Salter  *  illustrated  and  described  an  eye-tubercle  on 
each  cheek  of  Trinudeus,  from  which  there  was  a  raised  line 
extending  obliquely  upward  to  a  punctum  or  spot  on  each 
side  of  the  glabella.  He  considered  this  line  as  a  discontinu- 
ous facial  suture,  but  the  true  suture  was  afterward  correctly 
determined  by  Barrande,  f  and  in  well-preserved  specimens 
may  be  easily  observed  extending  around  the  entire  frontal 
and  lateral  border  of  the  head,  and  including  the  genal 
spines.  The  "eye-line  "  was  further  recognized  by  McCoy,  $ 
and  made  one  of  the  bases  for  a  division  of  the  genus  into 
two  sections  or  genera,  —  Trinudeus  proper  and  Tetraspis. 
These  divisions  were  accepted  by  Salter,  but  later  were  thor- 
oughly discussed,  and  rejected  by  Barrande  (I.  c.,  p.  617), 
upon  valid  grounds.  Nicholson  and  Etheridge,  §  in  1879, 
reviewed  these  facts  at  some  length,  and  gave  original  figures 
illustrating  the  ocular  tubercle  and  eye-line.  They  also 
agree  with  Barrande  in  recognizing  them  as  clearly  adoles- 
cent characters. 

The  justice  of  these  conclusions  is  substantiated,  and  addi- 
tional results  are  reached,  from  the  study  of  a  series  of 
Trinudeus  concentricus  Eaton,  found  associated  with  Triar- 
thrus  Becki  Green,  in  the  Utica  slate,  near  Rome,  New  York. 
The  remarkable  preservation  of  the  fossils  at  this  locality 
has  already  afforded  a  means  of  determining  all  the  principal 
details  of  the  ventral  structure  of  the  trilobite  genus  Triar- 
thrus,  and  there  is  now  distinct  evidence  as  to  the  nature  of 
the  appendages  in  another  type,  —  Trinudeus,  as  well  as  to 
the  probable  significance  of  the  so-called  "  eye-tubercle." 

*  On  the  structure  of  Trinudeus,  with  Remarks  on  the  Species.  Quar.  Jour. 
Geol.  Soc.,  Ill,  251-254. 

t  Syst.  Sil.  Boheme,  I,  1852. 

J  Ann.  Mag.  Nat.  Hist.,  2d  Series,  IV,  1849. 

§  Monograph  of  the  Silurian  Fossils  of  the  Girvan  District  in  Ayrshire,  fasc. 
ii,  1879. 


222  STUDIES  IN  EVOLUTION 

As  compared  with  Triarthru*,  specimens  of  Trinudeus  are 
not  very  common  at  this  locality,  and  although  more  than 
fifty  individuals  of  the  latter  have  been  obtained  from  the 
collections  presented  to  the  Yale  Museum  by  Professor  Marsh, 
not  more  than  half  a  dozen  of  these  are  adult  specimens,  and 
but  three  show  any  appendages.  Young  specimens  of  all  ages 
occur,  from  about  1  mm.  across  the  cephalon  upward,  and  in 
all  the  eye-line  and  eye-tubercle  are  present  until  a  width  of 
nearly  5  mm.  is  attained,  when  in  the  present  species  these 
features  dwindle  and  disappear,  leaving  no  discoverable  traces 
in  the  adult. 

Two  cephala  of  young  individuals,  without  the  free-cheeks, 
are  shown  enlarged  in  figures  1  and  2  of  Plate  X.  Figure  2 
represents  a  specimen  before  the  appearance  of  the  perforate 
border,  and  figure  1  gives  a  later  stage,  having  two  rows  of 
perforations  around  the  head.  On  both  specimens  the  eye- 
line  is  clearly  shown,  extending  somewhat  obliquely  back- 
ward from  the  anterior  lobe  of  the  glabella  to  the  central 
area  of  the  fixed-cheeks,  enlarging  slightly,  and  terminating 
in  a  rounded  node  or  tubercle  (a,  a,  figure  2). 

In  seeking  for  homologous  features  in  other  trilobites,  the 
genera  Harpes  and  Harpides  are  immediately  suggested, 
since  they  have  similar  oculai  ridges  extending  from  the  sides 
of  the  glabella,  and  ending  in  a  tubercle,  which,  in  Harpes^ 
contains  from  one  to  three  eye-spots,  as  determined  by  Bar- 
rande.  They  further  agree  in  having  these  visual  organs  on 
the  fixed-cheeks,  while  in  all  other  trilobites  with  distinct  eyes 
the  free-cheeks  carry  the  visual  areas.  This  type  of  eye 
is  thus  quite  different  in  its  relations  to  the  parts  of  the 
cephalon,  from  that  of  Phacops  or  Asaphus,  and  more  nearly 
resembles  the  eyes  of  some  of  the  Merostomata  (Bellinurus  ), 
as  do  also  the  triangular  areas  in  the  young  Trinucleus,  so 
distinctly  marked  off  from  the  fixed-cheeks  on  each  side 
of  the  glabella  behind  the  eye-line.  Adult  Trinucleus  and 
Harpes  have  these  areas  much  reduced  and  often  obsoles- 
cent. A  spot  or  node  in  the  median  line  on  the  glabella  has 
been  noticed  by  many  observers,  and  although  its  nature  has 


STRUCTURE   AND  APPENDAGES  OF  TRINUCLEUS     223 

not  been  demonstrated,  it  has  generally  been  called  an  ocellus. 
It  is  more  clearly  preserved  in  adult  specimens,  though  it  can 
be  detected  in  young  examples,  as  indicated  in  figures  1,  2, 
Plate  X. 

An  eye-line  occurs  in  many  early  trilobite  genera,  and  is 
well  marked  in  Conocoryphe,  Olenus,  Ptychoparia,  and  Arethu- 
sina.  At  least  four-fifths  of  the  Cambrian  forms  preserve 
this  feature,  which  is  almost  entirely  eliminated  before  Devo- 
nian time.  It  differs  in  extent,  but  not  necessarily  in  nature, 
from  the  eye-line  of  Trinucleus  and  Harpes  in  running 
entirely  across  the  fixed-cheeks  to  the  free-cheeks,  ending  in 
the  palpebral  lobe  in  eyed  forms.  It  is  evidently  a  larval 
character  in  the  trilobites,  as  shown  from  its  geological 
history  and  the  ontogeny  of  Trinucleus.  From  the  direction 
of  the  optic  nerve  in  Limulus^  and  its  relations  to  the  surface 
features  of  the  cephalothorax,  the  eye-line  probably  repre- 
sents the  course  of  that  nerve,  and  is  of  much  less  morpho- 
logical importance  than  the  different  types  and  arrangement  of 
visual  organs. 

The  pygidium  of  young  T.  concentricus  (Plate  X,  figure  3) 
is  remarkable  for  the  lack  of  definition  between  the  axis  and 
pleura.  In  later  and  adult  stages  the  number  of  ridges  on  the 
pleura  and  axis  do  not  correspond,  and  from  figures  4,  5,  and 
6  it  is  evident  that  in  this  genus  the  number  of  pleura  is 
no  indication  of  the  number  of  pygidial  segments  or  pairs  of 
appendages,  which,  however,  may  be  shown,  as  in  this  case, 
by  the  annulations  of  the  axis.  In  this  respect  the  pygidia 
in  Encrinurus,  Cybele,  and  Dindymene  are  of  the  same  nature. 
Figure  6  also  shows  a  narrow,  striated  doublure,  a  character 
generally  overlooked  in  descriptions  of  Trinucleus. 

Appendages. 

Three  specimens  have  been  thus  far  observed  which  show 
the  nature  of  the  appendages  in  Trinucleus.  Two  of  these 
are  illustrated  in  figures  4,  5,  and  6  of  Plate  X.  Figure  4 
represents  the  thorax  and  pygidium  viewed  from  the  dorsal 


224  STUDIES  IN  EVOLUTION 

side.  In  this  specimen  the  pyrite  which  replaced  the  chiti- 
nous  remains  of  the  animal  has  decomposed,  and  the  dorsal 
crust  weathered  away,  exposing  below  the  stems  of  the  exo- 
podites,  with  their  fringes  extending  over  the  entire  pleural 
areas  on  both  sides.  A  pygidium,  with  three  attached 
thoracic  segments,  from  another  entire  specimen  (figures  5 
and  6),  preserves  the  details  of  the  appendages  in  the  most 
perfect  and  satisfactory  manner.  As  both  halves  showed 
essentially  the  same  extent  and  disposition  of  the  fringes  on 
the  dorsal  side,  the  specimen  was  cut  in  two  along  the  centre 
of  the  axis,  and  the  left  side  was  then  embedded  in  paraf- 
fine.  By  careful  preparation  the  appendages  were  exposed 
from  the  ventral  side. 

The  cephala  of  the  three  specimens  described  are  consider- 
ably compressed,  and  from  them  a  very  imperfect  knowledge 
of  the  mouth  parts  could  be  obtained,  so  that  this  information 
must  be  left  to  future  discovery. 

Endopodites.  —  The  three  posterior  thoracic  endopodites 
are  very  similar,  and  in  a  general  way  closely  resemble  those 
of  Triarthrus  from  the  same  region  of  the  thorax.  They  are, 
however,  comparatively  shorter  and  stouter,  and  could  not  be 
extended  beyond  the  ends  of  the  pleura.  The  two  distal 
joints  are  cylindrical,  with  well-marked  articular  surfaces  and 
ridges.  The  joints  preceding  these  proximally  become  much 
wider,  flattened,  and  produced  into  transverse  extensions 
which  carry  large  tufts  of  setae  at  the  end,  as  also  does  the 
end  of  the  last  joint  of  the  limb  (dactylopodite). 

The  endopodites  on  the  pygidium  offer  no  conspicuous  dif- 
ferences from  those  just  described,  except  that  a  gradual 
change  in  form  is  manifest  as  the  terminal  limbs  are  reached. 
The  separate  endites  become  more  and  more  transversely 
cylindrical,  until  the  whole  limb  appears  to  be  made  up  of 
cylindrical  segments  transverse  to  its  length.  A  similar  con- 
dition was  observed  in  the  young  of  Triarthrus* 

Exopodites.  —  These  seem  to  be  composed  of  slender  joints, 
the  distal  exites  being  long  and  slightly  curved  outward. 

*  Amer.  Jour.  Sci.  (3),  XL VII,  pi.  vii,  fig.  3,  April,  1894. 


STRUCTURE  AND  APPENDAGES   OF  TRINUCLEUS    225 

They  carry  very  long,  close-set,  overlapping,  lamellose  fringes, 
which  evidently  had  a  branchial  function.  Some  of  the 
lamellae  are  spiniferous.  The  exopodites  become  shorter  on 
the  pygidium,  and  apparently  are  represented  near  the  end 
of  the  series  of  limbs  by  the  oval  plates  indicated  at  <?,  figure 
6.  If  this  interpretation  is  correct,  the  posterior  exopodites 
are  simple  flabella  attached  to  the  limbs,  as  in  Apus. 

Both  Professors  A.  E.  Verrill  and  S.  I.  Smith  agree  that 
the  characters  of  the  appendages  in  Trinucleus  indicate  an 
animal  of  burrowing  habit,  which  probably  lived  in  the  soft 
mud  of  the  sea-bottom,  much  after  the  fashion  of  the  modern 
Limulus.  In  addition  to  its  limuloid  form,  the  absence  of 
eyes  seems  to  favor  this  assumption.  So  does  the  fact  that 
many  specimens  have  been  found  preserving  the  cast  of  the 
alimentary  canal,  showing  that  the  animal  gorged  itself  with 
mud  like  many  other  sea-bottom  animals. 


15 


Ill 


STUDIES   IN  THE   DEVELOPMENT 
OF   THE   BRACHIOPODA 

1.  DEVELOPMENT  OF  THE   BRACHIOPODA 

2.  SOME  CORRELATIONS  OF  ONTOGENY  AND  PHYLOGENY 

IN  THE  BRACHIOPODA 

3.  REVISION  OF  THE  FAMILIES  OF  LOOP-BEARING  BRACH- 

IOPODA 

4.  DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA 

5.  DEVELOPMENT  OF  BILOB1TES 

6.  DEVELOPMENT  OF  TEREBRATALIA  OBSOLETA  DALL 

7.  DEVELOPMENT  OF  THE  BRACHIAL  SUPPORTS  IN  DIE- 

LASMA  AND  ZYGOSPIRA 


Ill 


STUDIES  IN  THE  DEVELOPMENT  OF 
THE  BRACHIOPODA* 

1.  DEVELOPMENT  OF   THE   BRACHIOPODA 
PART  I.   INTRODUCTION! 

(PLATE  XI) 

THE  Brachiopoda  have  been  so  carefully  studied  that  any 
new  general  conclusions  regarding  them  must  be  naturally 
based  upon  features  not  heretofore  considered.  In  other 
classes  of  animals  such  important  results  have  been  recently 
reached  by  the  application  of  the  law  of  morphogenesis  as 
defined  by  Hyatt,  that  the  writer  was  led  to  study  the 
Brachiopoda  from  this  standpoint.  The  facts  observed  by 
this  method  are  mainly  new  to  the  class,  and  considerably 
affect  the  taxonomic  positions  and  affinities  of  the  various 
families  and  genera. 

The  value  of  the  stages  of  growth  and  decline  in  work 

o  o 

relating  to  phylogeny  and  classification  is  now  generally 
admitted.  The  memoirs  of  Hyatt,  Jackson,  and  others 
amply  show  that  the  clearest  and  simplest  understanding  of 
a  group  may  be  thus  reached.  The  application  of  the  prin- 
ciples of  growth,  acceleration  of  development,  and  mechan- 

*  No  revision  of  these  papers  has  been  undertaken  further  than  to  bring  the 
nomenclature  of  genera  and  species  up  to  date;  also  the  current  terms  of 
auxology,  as  agreed  upon  by  Hyatt,  Buckman,  and  Bather,  have  been  substituted 
for  those  first  proposed. 

t  Amer.  Jour.  Sci.  (3),  XLI,  343-357,  pi.  xvii,  1891. 


I 


230  STUDIES  IN  EVOLUTION 

ical  genesis  form  the  main  factors  in  the  studies  here  made. 
The  geologic  sequence  of  genera  and  species  in  this  connec- 
tion is  also  of  the  greatest  importance,  for  in  this  way  the 
development  of  ancient  species  may  be  studied,  which  in 
their  adult  condition  represent  neanic  or  nepionic  stages  of 
later  forms. 

The  prolific  development  of  the  Brachiopoda,  both  in  point 
of  numbers  and  variety  of  genera  and  species,  together  with 
their  geological  history,  mark  this  group  as  one  which  should 
furnish  important  data  for  the  study  of  its  genesis  and  of  the 
limits  of  a  specialized  variation  in  a  single  class.  Moreover, 
as  its  culmination  was  reached  in  Paleozoic  time,  the  group 
should  afford  illustration  of  many  principles  of  evolution. 
x  The  main  characters  common  to  the  class  of  Brachiopoda 
are  as  follows :  The  bivalve  shell ;  the  pedicled  or  fixed  con- 
dition ;  the  animal  composed  of  two  pallial  membranes  inti- 
mately related  to  the  shell ;  a  visceral  sac ;  and  two  arms  or 
appendages  near  the  mouth.  The  extreme  range  of  variation 
does  not  eliminate  any  of  these  features,  and,  consequently, 
no  univalve  or  multivalve  forms  are  found,  nor  any  strictly 
free-swimming  species,  nor  growths  or  modifications  adapting 
the  organism  to  a  pelagic  life.  Thus  the  limits  of  modifica- 
tion are  narrowly  restricted  as  compared  with  those  of  several 
other  classes;  namely,  the  Echinodermata  and  the  Pelecypoda, 
but  the  thousands  of  known  species  of  Brachiopoda  show 
what  differentiation  has  taken  place  within  these  limits. 

The  Protegulum. 

The  first  important  observation  to  be  noted  is  that  all 
brachiopods,  so  far  as  studied  by  the  writer,  have  a  common 
form  of  embryonic  shell,  which  may  be  termed  the  protegu- 
lum.*  The  protegulum  is  semi-circular  or  semi -elliptical  in 
outline,  with  a  straight  or  arcuate  hinge -line,  and  no  hinge- 
area.  A  slight  posterior  gaping  is  produced  by  the  pedicle 
valve  being  usually  more  convex  than  the  brachial.  The 

*  From  irpda  early,  and  rtyos  a  covering. 


DEVELOPMENT  OF  THE  BRACHIOPODA  231 

modifications  noted  are  apparently  due  to  accelerated  growth, 
by  which  characters  primarily  neanic  become  so  advanced  in 
the  development  of  the  individual  as  to  be  impressed  finally 
upon  the  embryonic  shell.  This  feature  is  well  shown  in  the 
development  of  Orbiculoidea  and  Discinisca,  and  is  reserved 
for  discussion  under  these  genera. 

As  the  protegulum  has  been  observed  in  about  forty  genera* 
representing  nearly  all  the  leading  families  of  the  class,  its_ 
general  presence  may  be  safely  assumed.  In  size  it  varies  / 
in  different  genera  and  species.  The  range  is  from  .05  to 
.60  mm.  A  similar  range  in  the  prodissoconch  of  pelecypods 
has  been  noticed  by  Dr.  Robert  T.  Jackson.  The  protoconch 
of  cephalopods  and  gastropods  also  varies  greatly.  In  all 
these  classes  the  size  of  the  initial  shell  has  no  special 
relation  to  the  mature  form,  and  it  seems  to  have  little 
significance  in  related  genera  or  species. 

The  structure  of  the  protegulum  has  been  described  as 
corneous  and  imperforate.  In  all  probability  it  is  the  same 
for  the  entire  class,  whether  among  the  corneous  and  phos- 
phatic  linguloids  and  discinoids,  or  the  terebratuloids  and 
other  forms  having  carbonate  of  calcium  shells.  Professor 
E.  S.  Morse,  in  describing  the  early  stages  of  Terebratulina,^ 
says :  "  A  heart-shaped  corneous  shell  is  formed  even  at  this 
early  stage,  for  in  several  cases  I  met  with  it  where  the  softer 
portions  had  been  removed  by  Paramaecia."  Similarly,  in 
the  genus  Cistella  according  to  Kovalevski:  :f  "En  m§me 
temps  la  coquille  se  forme,  par  suite  du  de*pot  sur  la  cuticule 
chitineuse  des  minces  couches  de  calcaire,  dans  lesquelles  on 

*  Atretia  (Cryptopora),  Chonetes,  Cistella,  Conotreta,  Crania,  Craniella,  Discina, 
Discinisca,  Glottidia,  Gwynia,  Kraussina  (Megerlina),  Laqueus,  Leptcena,  Lingula, 
Lingulops,  Linnarssonia,  Liothyrina,  Magellania  (Macandrevia),  Martinia,  Muhl- 
feldtia,  Obolus?  (Ehlertella,  Orbiculoidea,  Orthis  group,  Pholidops,  Productella, 
RhynchoneJJa  (Hemithyris),  Schizambon,  Schizobolus,  Schizocrania,  Schizotreta, 
Spirifer,  Streptorhijnchus  (Orthothetes),  Stropheodonta,  Strophomena,  Terebratella, 
Terebratulina,  Thecidium,  (Lacazella),  Trematis,  Tropidoleptus,  Zt/gospira. 

t  Embryology  of  Terebratulina.  Mem.  Boston  Soc.  Nat.  Hist.,  II,  257,  vide 
figs.  68,  76,  pi.  viii,  1873. 

J  Developpement  des  Brachiopodes,  Kovalevski.  Analyse  par  MM.  (Ehlert 
etDeniker,  65.  67,  1883. 


232  STUDIES  IN  EVOLUTION 

ne  voit  point  encore  les  perforations  tubulaire."  Previous 
to  this  stage,  "  Les  lobes  du  manteau  commencent  alors  a  se 
recouvrir  d'une  cuticule  dpaisse  et  rigide  que  ne  leur  permet 
plus  de  se  mouvoir  que  dans  le  sens  vertical." 

From  the  minuteness  and  the  tenuous  nature  of  the  pro- 
tegulum,  its  fossil  preservation  in  an  unaltered  condition 
would  not  be  expected.  Neither  would  it  be  found  on  the 
beaks  of  mature  shells,  whether  recent  or  fossil.  In  rare 
cases  of  unusually  perfect  conservation  of  the  beaks  the  pro- 
tegulum  is  retained,  but  frequently  its  form  and  characters 
are  exhibited,  after  its  removal,  by  the  impression  left  in  the 
surrounding  calcareous  test.  To  study  the  features  of  the 
protegulum,  and  the  early  stages  in  the  growth  of  the  shell, 
it  is  very  desirable  and  often  necessary  to  have  young  and 
well-preserved  specimens.  The  rapid  encroachment  of  the 
pedicle  on  the  ventral  beak  commonly  obliterates,  at  an 
early  period,  all  traces  of  the  protegulum  and  early  nepionic 
stages;  while,  in  the  dorsal  valve,  abrasion  from  foreign 
objects,  or  against  the  deltidial  covering,  or  the  pedicle 
itself  usually  removes  all  early  lines  of  growth  or  nepionic 
characters.  In  general,  fully  matured  shells,  recent  or 
fossil,  do  not  furnish  material  for  a  study  of  the  incipient 
growth -stages. 

Affinities.  —  In  looking  for  a  prototype  preserving  through- 
out its  development  the  main  features  of  the  protegulum, 
and  showing  no  separate  or  distinct  stages  of  growth,  the 
early  primordial  form  hitherto  known  as  Kutorgina  Billings 
is  at  once  suggested.  This  genus,  as  shown  below,  includes 
two  distinct  types,  for  one  of  which  the  name  Paterina 
[=  Iphidea  Walcott]  is  proposed.* 

*  The  strict  definition  of  Kutorgina  limits  it  to  calcareous  shells  such  as  are 
found  near  Swanton,  Vermont,  often  occurring  as  casts  in  the  limestone.  The 
original  description  of  Obolella  cingulata  by  Billings  (Geology  of  Vermont,  II. 
948,  figs.  347-349,  1861)  seems  to  include  two  species.  One,  represented  by 
figures  347  and  349  (he.  cit.),  agrees  with  phosphatic  species  having  a  straight 
hinge-line  as  long  as  the  width  of  the  shell.  The  other,  shown  in  figure  348,  has 
a  calcareous  test,  shorter  hinge,  flattened  brachial  valve,  and  convex  pedicle 
valve  with  arching  beak.  Upon  the  latter  species  the  genus  was  founded,  and  it 


DEVELOPMENT  OF  THE  BRACHIOPODA  233 

The  valves  of  Paterina  [—  Iphidea]  are  sub-equal,  the 
pedicle  valve  being  a  little  more  elevated  than  the  brachial. 
They  are  semi-elliptical  in  outline.  In  mature  specimens 
all  lines  of  growth,  from  the  nucleal  shell  to  the  margin,  are 
unvaryingly  parallel  and  concentric,  terminating  abruptly  at 
the  cardinal  line.  In  other  words,  no  changes  occur  in  the 
outlines  or  proportions  of  the  shell  during  growth,  through 
the  nepionic  and  neanic  stages  up  to  and  including  the 
completed  ephebic  condition.  The  resemblance  of  this  form 
to  the  protegulum  of  other  brachiopods  is  very  marked 
and  significant,  as  it  represents  a  mature  type  having  only 
the  common  embryonal  features  of  other  genera.  It  is  of 
further  importance  as  representing,  in  many  species,  an 
early  condition  of  nepionic  growth  subsequent  to  the  pro- 
tegulum, during  which  the  proportions  and  features  of 
the  shell  undergo  no  modification  except  increase  in  size. 
This  is  termed  the  Paterina  stage.  It  is  well  shown  in 
the  brachial  valve  Orbiculoidea  minuta  Hall  (Plate  XI, 
figure  5).  * 

Modifications  from  Acceleration.  —  The  modifications  in  the 
form  of  the  protegulum  are  due  to  the  influence  of  accelerated 

has  been  recognized  as  the  type  by  C.  D.  Walcott  (Bull.  U.  S.  Geol.  Surv., 
No.  30,  102,  pi.  ix,  figs.  1,  1  a,  b,  1886).  The  species  represented  by  Billings  in 
figures  347  and  349  resembles  Obolus  labradoricus  (fig.  345,  loc.  cit.},  and  is  repre- 
sented by  Walcott  (loc.  cit.,  pi.  ix,  figs.  2,  2  a,  b)  and  referred  by  him  also  to 
Kutorgina.  Mr.  Walcott  recognizes  two  groups  of  species,  which  are  classified 
(p.  102)  as:  "shell  structure  calcareous  ( K.  cingulata,  K.  Whitfieldi)  or  horny 
(K.  Labradorica,  K.  sculptilis) ." 

An  examination  of  specimens  representing  both  groups  leads  the  writer  to 
consider  Kutorgina  cingulata  and  Obolus  labradoricus  of  Billings  as  generically 
distinct.  Therefore  the  name  Paterina  is  here  proposed  to  include  species  of  the 
type  of  Obolus  labradoricus,  var.  swantonensis.  This  name  is  intended  to  express  the 
primitive  ancestral  characters  which  it  possesses  (Plate  XI,  figs.  1,2).  Exfoliated 
specimens  of  Paterina  labradorica  show  a  roughened  area  on  the  cast,  each  side 
of  the  median  line  near  the  beak.  These  probably  represent  muscular  attach- 
ments. Sections  of  the  shell  show  no  hinge-area  as  described  in  K.  cingulata. 
A  study  of  the  latter  would  doubtless  present  distinct  stages  of  growth.  The 
dissimilar  valves,  arcuate  ventral  beak,  and  mesial  depression  could  be  developed 
only  by  passing  through  several  well-marked  phases.  This  in  itself  seems 
sufficient  for  a  separation  were  no  other  characters  present.  [Walcott  has 
reinvestigated  the  genus  Iphidea,  and  has  decided  that  it  includes  the  form 
above  designated  as  Paterina.  Proc.  U.  S.  Nat.  Mus.,  XIX,  707,  1897.] 


234  STUDIES  IN  EVOLUTION 

growth,  by  which  nepionic  and  sometimes  neanic  features  are 
pushed  forward,  or  appear  earlier  in  the  history  of  the  indi- 
vidual, so  as  to  become  impressed  upon  the  early  embryonic 
shell.  Only  a  brief  review  of  these  changes  -will  be  noted 
Jiere,  as  a  fuller  description  properly  belongs  under  the  dis- 
cussions of  the  various  genera  and  families.  Naturally,  the 
greatest  departure  from  the  normal  protegulum  is  exhibited 
in  the  most  variable  and  specialized  valve,  the  ventral  valve. 
The  nearly  equivalve  genera,  as  Lingula  and  G-lottidia,  pre- 
sent almost  no  modification.  In  the  ventral  valve  of  Linnars- 
sonia  and  Orbiculoidea  (Plate  XI,  figure  7),  the  protegulum 
has  a  hinge  more  or  less  arcuate.  Discinisca  shows  a  sub- 
circular  ventral  protegulum  with  a  pedicle-notch,  and  the  evi- 
dence of  any  hinge  in  the  dorsal  valve  is  very  slight  (Plate  XI, 
figures  8,  9).  The  discinoid  character  appearing  in  the  second 
and  third  nepionic  stage  of  the  Paleozoic  Orbiculoidea  (Plate 
XI,  figure  6)  has  become  so  accelerated  in  neozoic  and  recent 
Discinisca  as  to  produce  a  discinoid  protegulum. 

The  strophomenoid  shells  usually  retain  a  normal  protegu- 
lum in  the  dorsal  valve,  but  in  the  ventral  valve  the  pro- 
tegulum has  an  abbreviated  hinge  and  arcuate  hinge -line 
(Plate  XI,  figures  13,  14,  15). 

No  marked  variation  has  been  yet  observed  among  the  spire - 
bearing  genera,  nor  has  any  been  seen  in  the  terebratuloids 
or  rhynchonelloids  further  than  the  radii  on  the  protegulum 
of  Atretia  (Cryptopora).  Possibly  this  feature  in  Atretia  is 
an  inheritance  from  the  radiate  character  of  the  shell  in  the 
Rhynchonellidse.  It  may  be,  however,  one  of  the  features 
consequent  upon  its  fragile  nature  and  deep-sea  habitat,  as 
observed  among  other  abyssal  shells. 

Differences  in  the  Valves. 

The  dissimilarity  in  the  form  and  relations  of  the  two 
valves  progressively  increases  in  the  following  genera :  Lin- 
gula, Terebratulina,  Cistella,  Discinisca,  Thecidium  (Lacazella), 
and  Crania.  Lingula  is  nearly  equivalve,  both  valves  bear- 
ing a  close  resemblance  to  each  other.  In  Terebratulina  and 


DEVELOPMENT  OF  THE  BRACHIOPODA  235 

Cistella  the  two  valves  are  more  strongly  specialized,  while 
in  Discinisca,  Thecidium,  and  Crania  they  are  quite  unlike. 

Two  important  organic  characters  accompany  and  partake 
of  a  similar  amount  of  variation;  (a)  the  length  and  direc- 
tion of  the  pedicle,  and  (6)  the  position  and  structure  of  the 
pedicle-opening.  Lingula  with  a  long,  fleshy,  mobile  pedicle 
receives  uniformly  disposed  axil  impacts  on  the  valves,  and 
therefore,  with  equal  physiological  reactions,  equality  in  size 
and  form  is  produced.  Terebratulina  and  most  of  the  other 
terebratuloids  and  rhynchonelloids  have  a  shorter  and  less 
flexible  pedicle.  As  a  whole  the  motions  of  the  animal  are 
more  restricted;  the  pedicle-opening  is  confined  mainly  to 
one  valve;  the  valves,  consequently,  are  differently  related 
to  the  environment,  and  express  this  difference  in  their  dis- 
similarity. In  these  examples,  also,  the  inclination  of  the 
pedicle  to  the  longitudinal  axis,  or  of  the  shell  to  the  surface 
of  support,  agrees,  pari  passu,  with  the  amount  of  unlikeness 
in  the  valves,  except  when  the  pedicle  is  so  shortened  as  to 
interfere  with  their  free  movement.  To  this  inclination  is 
probably  due  the  difference  in  the  action  of  the  forces  from 
without. 

Normally,  in  Lingula,  the  pedicle  is  in  direct  linear  con- 
tinuation with  the  axis  of  the  shell.  Terebratulina  and 
Magellania  are  inclined  at  an  angle  of  40°  to  the  surface  of 
support,  but  in  Cistella  and  Muhlfeldtia  this  is  increased  to 
about  70°.  In  the  latter  genera,  although  the  position  of 
the  axis  is  nearly  vertical,  the  shortening  of  the  pedicle  pre- 
cludes more  than  a  slight  elevation  and  rotation  of  the  organ- 
ism. The  more  the  pedicle-opening  is  confined  to  one  valve 
the  greater  is  the  difference  between  both. 

Passing  to  Disuinisca,  the  pedicle  is  found  to  be  at  right 
angles  to  the  longitudinal  axis,  and  the  valves  become  strictly 
an  upper  and  a  lower.  The  lower  rests  upon  the  object  of 
support,  and  the  animal  is  capable  of  raising  and  rotating  it 
only  to  a  slight  degree.  Under  such  circumstances  the 
lower  valve  is  wholly  different  in  its  relations  to  the  environ- 
ment, and,  naturally,  it  expresses  the  greatest  dissimilarity 


236  STUDIES  IN  EVOLUTION 

in  the  two  valves  of  any  genus  yet  discussed.  In  some 
allied  genera,  as  Discina  (type  D.  striata)  and  Schizotreta, 
where  the  pedicle  is  small  and  the  lower  valve  rises  above 
the  object  of  support,  a  similar  form  in  both  valves  is  again 
produced  by  the  conical  growth  of  the  lower  valve. 

More  primitive  types,  as  Acrotreta  and  Acrothele,  having 
the  plane  of  the  dorsal  valve  at  right  angles  to  the  direction 
of  the  pedicle,  retain  a  marginal  upper  beak,  while  the  lower 
is  elevated,  sub-central,  and  perforate.  These  features  in 
Acrotreta  and  Discina  resemble,  in  a  measure,  those  in  the 
Rudistes.  In  Acrotreta,  as  in  Caprotina,  the  upper  valve 
shows  its  normal  affinities,  while  the  other  has  become 
highly  modified  and  dissimilar.  But  in  Discina  and  Hippu- 
rites  the  hinge-line  is  lost,  and  the  apex  of  the  upper  valve 
is  sub-central.  This  conical  habit  of  growth  in  erect  attached 
organisms  has  been  explained  as  the  physiological  reaction 
from  equal  radial  exposure  to  the  environment.  It  consti- 
tutes the  law  of  radial  symmetry,  ably  discussed  by  Haeckel, 
Jackson,  Korshelt,  and  Heider.  Its  application  to  the 
Brachiopoda  can  be  made  mainly  in  forms  having  the  pedicle 
perforation  sub-centrally  located  in  the  lower  valve. 

In  Thecidium  and  Crania  the  calcareous  union  of  the  lower 
valve  to  the  object  of  support  represents  the  extreme  of 
unlike  conditioning,  and  such  forms  exhibit  the  greatest 
difference  in  the  features  of  the  opposite  valves.  Crania, 
being  probably  derived  from  discinoid  stock,  is  without  proper 
hinge.  In  the  history  of  its  development,  so  far  as  known, 
it  does  not  show  beyond  the  protegulum  an  early  hinged  con- 
dition. Hence  there  is  no  indication  of  direct  derivation 
from  hinged  forms.  A  false  hinge  is  sometimes  present,  but 
it  clearly  shows  a  secondary  mechanical  adaptation,  and  not 
a  phylogenetic  character.  On  the  other  hand,  true  hinged 
attached  genera,  such  as  Thecidium  (Lacazella),  Davidsonia, 
and  Strophalosia,  possess  this  feature  as  a  later  ancestral 
character,  and  in  their  chronological  history  tend  to  shorten 
and  gradually  eliminate  it.  An  illustration  of  this  is  seen 
in  the  succession  of  the  species  in  Strophalosia,  or  in  the 


DEVELOPMENT  OF  THE  BRACHIOPODA  237 

ontogeny  of  one  of  the  Permian  species.  Strophalosia  Gold- 
fussi,  in  early  neanic  stages,  has  a  hinge-line  about  equal 
to  the  width  of  the  shell,  but  in  mature  individuals  it  is 
usually  less  than  one-half  the  width.  This  reduction  of  the 
hinge  and  ostrean  form  of  growth  are  in  accordance  with  the 
deductions  and  observations  made  upon  the  Oyster  and  its 
allies  by  Jackson,  and  the  mechanical  principles  are  evidently 
the  same  in  both  cases. 

One  of  the  most  conspicuous  examples  of  a  difference  in 
the  form  of  the  valves  is  shown  in  the  abnormal  genus  Pro- 
boscidella.  In  early  neanic  stages  it  resembles  an  ordinary 
Productus.  Afterward,  probably  from  burrowing  in  the 
mud,  the  ventral  valve  becomes  extravagantly  developed 
anteriorly  into  a  calcareous  tube.  This  is  accomplished  by 
the  excessive  growth  of  the  anterior  and  lateral  margins. 
Then  an  infolding  takes  place  until  the  lateral  edges  unite, 
after  which  the  tube  is  built  up  by  concentric  increment 
around  the  free  end.  The  resemblance  of  Proboseidella  to 
Aspergillum  is  quite  marked,  except  that,  in  the  latter  genus, 
the  tube  is  formed  from  the  growth  and  union  of  two  valves 
instead  of  one. 

From  the  morphological  differences  of  the  ventral  and 
dorsal  valves  it  will  be  seen  that  the  highest  modifications 
occur  in  the  former;  while  the  variations  in  the  latter  are 
expressed  mainly  as  adaptive  reactions  or  accommodations  to 
these  changes.  The  explanation  of  the  fact  that  greater 
alteration  takes  place  in  the  ventral  valve  evidently  lies  not 
in  the  greater  plasticity  of  this  member,  but  in  its  more 
highly  specialized  and  differentiated  external  form,  and 
mainly  in  its  being  the  lower  and  attached  valve. 

No  account  is  taken  here  of  the  crura,  loops,  and  spires  of 
the  brachial  valve,  so  characteristic  and  important  in  many 
families  and  genera.  These  are  evidently  processes  devel- 
oped by  the  internal  requirements  of  the  animal,  and  are  not 
affected  by  the  environment.  Therefore  they  are  internal 
calcined  organs  independent  of  the  form  or  manner  of  growth 
of  the  external  covering.  This  is  shown  by  the  fact  that  in 


238  STUDIES  IN  EVOLUTION 

each  group  there  is  a  frequent  recurrence  of  similar  general 
external  features,  whether  in  crurate,  looped,  or  spire-bearing 
genera. 

Genesis  of  Form. 

The  principal  characters  shared  by  the  two  valves  are  the 
general  outline  and  the  hinge.  In  typical  and  generalized 
forms,  as  Lingula^  Terebratulina^  Cistella,  and  Discinisca, 
considered  as  before  in  regard  to  length  of  pedicle,  freedom 
of  movement,  and  direction  of  longitudinal  axis  to  the  object 
of  support,  we  find  a  key  to  these  types  of  structure.  In  the 
individual  development  of  Terebratulina,  as  shown  by  Morse, 
we  first  have  the  early  embryonic  shell  (protegulum),  with 
a  short  pedicle  and  straight  hinge.  The  next  stage  retains 
both  these  characters,  but  the  valves  have  become  more  un- 
equal and  the  pedicle-opening  confined  to  the  delthyrium  of 
one  valve.  The  result  is  a  shell  very  much  like  Argiope  or 
Megerlia  (Megathyru  and  Muhlfeldtia)^  to  which  Professor 
Morse  also  called  attention.  The  same  author  next  showed 
that  the  succeeding  stage  had  a  comparatively  long  pedicle, 
and  a  shell  linguloid  in  form.  Afterward  the  defining  of 
the  pedicle-opening,  shortening  of  the  pedicle,  and  truncation 
of  the  ventral  beak  produced  the  final  characteristic  external 
features  of  Terebratulina.  The  deduction  from  this  example 
and  from  Lingula  is  that  genera  having  pedicles  sufficiently 
long  to  admit  of  freedom  of  axial  movement  have  elongate 
and  rostrate  shells.  The  shortening  of  the  pedicle  brings  the 
posterior  part  of  the  shell  in  more  or  less  close  proximity  to 
the  object  of  support,  and,  as  growth  cannot  take  place  in 
that  direction,  it  increases  laterally,  resulting  in  broader 
forms  with  extended  hinge-areas,  as  in  many  species  of 
Cistella,  Scenidium,  Muhlfeldtia,  Terebratella,  Kraussina,  etc. 

The  variety  known  as  Muhlfeldtia  truncata,  v'ar.  monstruosa 
Davidson,  further  shows  how  discinoid  characters  may  be 
produced  in  an  entirely  different  type  of  shell.  A  specimen 
was  found  by  the  writer  in  a  position  which  readily  gave  the 
solution  to  its  variation  from  the  normal  species.  It  was 


DEVELOPMENT  OF  THE  BRACHIOPODA  239 

attached  to  a  foreign  object  under  the  hinge-line  of  a  large 
mature  specimen  of  M.  truncata,  thus  forcing  the  axis  and 
plane  of  the  valves  into  parallelism  with  the  object  of  sup- 
port. In  this  way  the  pedicle  emerged  at  right  angles  to  the 
axis.  The  growth  of  the  shell  and  the  increase  in  the  size 
of  the  pedicle  caused  the  latter  to  encroach  on  the  substance 
of  the  lower  beak,  forming  a  dorsal  perforation  or  pedicle- 
notch,  which  in  this  example  amounted  to  an  arc  of  180°. 
As  the  ventral  valve  was  the  upper  and  the  dorsal  the  lower, 
with  the  pedicle -opening  through  the  latter,  only  the  abnor- 
mal position  of  the  shell  can  account  for  this  anomalous 
discinoid  condition.  In  the  development  of  Orbiculoidea,  a 
true  discinoid  genus,  it  will  be  seen  that  during  the  early 
stages  it  had  a  straight  hinge  and  marginal  beaks  (Plate  XI, 
figures  5,  6,  7).  Then,  from  its  procumbent  position  and 
peripheral  growth,  the  pedicle  became  more  and  more  en- 
closed by  the  lower  valve,  until  in  Schizotreta  (figure  11)  and 
Acrothele  (figure  12),  the  opening  finally  became  sub-central. 

The  resemblance  between  this  form  of  growth  and  habit 
and  Anornia  is  very  suggestive.  Morse  and  Jackson  have 
shown  that  from  an  early  normal,  bivalve,  hinged  shell  the 
right  valve  in  its  subsequent  growth  surrounds  the  byssus, 
which  occupies  much  the  same  position  and  performs  a  func- 
tion similar  to  the  pedicle  of  Discinisca  and  Orbiculoidea. 
Peripheral  growth  also  causes  the  initial  shell  to  recede  from 
the  margin.  Another  instance  is  thus  furnished  of  a  dis- 
cinoid habit  in  an  organism  otherwise  entirely  different.  It 
is  therefore  evident  that  the  discinoid  form  is  purely  due 
to  the  mechanical  conditions  of  growth.  Hence  the  writer 
believes  that  any  bivalve  shell  with  the  plane  parallel  to  the 
object  of  support,  and  attached  by  a  more  or  less  flexible, 
very  short  organ,  as  a  byssus  or  a  pedicle,  without  calcareous 
cementation,  assumes  a  discinoid  mode  of  growth. 

The  conditions  of  radial  symmetry  and  ostrean  growth 
were  briefly  mentioned  in  a  preceding  section,  and  need  only 
be  cited  here  as  resulting  from  the  cemented  state  of  fixation, 
as  shown  in  species  of  Thecidium,  Strophalovia,  and  Crania. 


240  STUDIES  IN  EVOLUTION 

A  long  pedicle  accompanies  elongate  shells  with  short 
hinges.  A  short  pedicle  causes  extended  hinge  growth  when 
the  plane  of  the  valves  is  ascending  or  vertical,  but  a  dis- 
cinoid  form  results  when  the  plane  of  the  valves  is  horizontal. 

Types  of  Pedicle-openings. 

M.  Deslongchamps  is  one  of  the  few  writers  who  have 
given  much  consideration  to  the  characters  of  the  pedicle- 
opening.  His  studies,  although  mostly  confined  to  the  tere- 
bratuloids  and  later  spire-bearing  genera,  conclusively  show 
the  importance  of  this  feature.*  In  a  recent  paper  by  the 
writer  f  attention  was  called  to  the  persistence  and  embryonic 
features  of  this  portion  of  the  shell.  "  It  has  been  shown  by 
J.  M.  Clarke  and  the  writer  that  all  species,  so  far  as  exam- 
ined, possessing  a  true  deltidium  [=  deltidial  plate]  in  the 
adult  state,  show  that  it  was  gradually  developed  in  early 
stages  of  growth,  by  concrescence  along  the  lateral  margins 
of  an  open  triangular  area.  Also,  that  all  species  furnished 
with  a  pedicle-sheath  [=  deltidium]  have  it  fully  developed 
in  the  earliest  growth-stages  which  have  been  observed  for 
these  species,  and  the  subsequent  growth  of  the  individual 
does  not  materially  alter  its  general  characters,  except  that 
it  is  sometimes  retrogressive,  the  parts  becoming  atrophied  or 
functionally  obsolete.  A  feature  of  such  importance,  and  so 
intimately  connected  with  the  embryonal  growth  of  the  shell, 
must  be  given  considerable  significance  in  discussing  the 
various  genera  in  which  it  is  present  or  absent."  At  that 
time  the  development  and  true  interpretation  of  these  differ- 
ent features  of  the  pedicle-opening  and  the  early  stages  of 
the  shell  had  not  been  studied  sufficiently,  and  a  more  gen- 
eral application  of  the  principles  involved  could  not  then  be 
made.  The  results  of  later  studies  give  prominence  to  these 
characters,  and  show  that  they  furnish  a  method  for  an 
ordinal  grouping  of  the  genera  of  brachiopods.  This  is 

*  Note  sur  le  developpement  du  deltidium  chez  les  brachiopodes  articules. 
Bull.  Soc.  Ge'ol.  France  (2),  XIX,  409-413,  pi.  ix,  1862. 
t  Amer.  Jour.  Set.  (3),  XL,  217,  September,  1890. 


DEVELOPMENT  OF  THE  BRACHIOPODA  241 

found  to  agree  with  the  chronological  history  of  the  class,  as 
well  as  with  the  anatomical  and  shell  characters,  and  there- 
fore it  is  believed  to  be  a  natural  and  reliable  sub-division. 

The  first  and  simplest  type  of  pedicle-opening  is  in  shells 
with  a  posterior  gaping  of  the  valves,  through  which  the 
pedicle  protrudes  in  line  with  the  axis.  It  is  shared  more  or 
less  by  both  valves,  although,  generally,  the  greater  portion 
of  the  periphery  is  included  by  the  pedicle  valve.  The 
genus  Lingula  and  related  genera  afford  types  of  this  form 
of  pedicle -opening. 

The  second  type  is  characterized  by  a  pedicle  wholly  con- 
fined to  the  lower  valve,  and  emerging  at  right  angles  to  the 
plane  of  the  valves.  In  primary  forms  it  is  not  entirely 
surrounded  by  shell  growth,  but  occupies  a  sinus,  slit,  or 
fissure.  A  further  specialization  carries  it  quite  within  the 
periphery,  and  it  finally  becomes  sub-central.  A  serial  illus- 
tration of  this  type  is  presented  in  the  genera  Schizocrania, 
OrMculoidea,  Discinisca,  Schizotreta,  and  Acrothele.  The 
group  probably  terminates  with  forms  like  Crania  and  PJwli- 
dops^  as  shown  by  the  development  of  the  dorsal  valve  and 
from  internal  characters.  The  development  of  the  lower 
valve,  however,  has  not  been  observed  as  yet  in  either  of 
these  genera. 

The  third  form  somewhat  resembles  the  second.  During 
the  first  nepionic  stage  of  shell  growth  the  pedicle  is  en- 
closed by  the  ventral  valve  and  the  pro-deltidium.  The  per- 
foration remains  sub-marginal,  and  does  not  tend  to  become 
centralized  as  in  the  preceding  group.  The  initial  pedicle - 
opening  may  be  maintained  by  further  growth,  forming  a 
deltidium;  or  it  may  be  merged  into  the  hinge  opening  by 
resorption  of  the  shell  or  by  pedicle  abrasion.  Orthisina, 
Leptcena,  Strophomena,  Chonetes,  and  Stropheodonta  furnish 
illustrations  of  the  first  condition,  and  the  second  is  repre- 
sented in  the  groups  of  Orthis. 

The  fourth  type  in  its  incipient  stage  marks  a  return  to 
the  simple  conditions  of  the  first,  but  in  early  neanic  stages 
the  pedicle  is  confined  to  the  ventral  beak,  and  deltidial 

16 


242  STUDIES  IN  EVOLUTION 

plates  are  developed  in  the  majority  of  species.  These  plates 
at  maturity  may  entirely  limit  the  pedicle-opening  below,  so 
that  the  pedicle  emerges  immediately  under  the  beak,  or 
encroaches  upon  the  substance  of  the  beak  itself.  This  type 
of  opening  is  shown  by  Zygospira,  Spirifer,  Rhyndionella, 
Terebratulina,  Magellania^  etc. 

The  only  divisions  of  the  class  which  have  had  continued 
recognition  are  the  Arthropomata  and  Lyopomata,  proposed 
by  Owen  in  1858.*  Subsequently  various  authors  gave  names 
to  express  other  characters,  but  all  included  the  same  ele- 
ments in  the  two  divisions.  Professor  Huxley's  terms,  the 
Articulata  and  Inarticulata,  have  also  come  into  current  use, 
and  are  convenient  to  express  the  nature  of  the  union  of  the 
valves.  All  the  names  proposed  for  these  divisions  by  Owen, 
Bronn,  Huxley,  Gill,  and  King,  are  based  upon  (1)  the  in- 
testinal canal  whether  ending  in  an  anus  or  in  a  blind  sac, 
(2)  the  relative  proportions  of  the  viscera  and  brachia  to  the 
shell  cavity,  and  (3)  the  character  of  the  union  of  the  valves. 

If,  as  Agassiz  has  said,f  orders  should  be  founded  upon 
facts  of  development  or  embryology,  the  ordinal  division 
into  groups  expressing  the  genesis  of  an  important  common 
character  should  furnish  a  satisfactory  classification.  The 
Articulata  and  Inarticulata  do  not  appear  to  have  a  primary 
developmental  basis  in  nature.  These  names  may  be  con- 
veniently retained  as  two  divisions  or  sub-classes,  but  they 
fail  to  express  the  true  relationships  of  the  various  groups 
included  in  them. 

In  1883  Dr.  Waagen  (Palseontologia  Indica)  proposed  a 
classification  comprising  six  sub-orders,  founded  partly  on  the 
pedicle-opening  and  on  the  form  of  the  brachial  supports. 
Two  of  his  groups,  the  Mesokaulia  and  Aphaneropegmata, 
are  nearly  equivalent  in  extent  to  the  Atremata  and  Pro- 
tremata  now  proposed.  Daikaulia  and  Gasteropegmata  of 
Waagen  are  here  included  in  the  Neotremata,  and  the  Telo- 

*  Encycl.  Brit.,  8th  ed.,  XV,  301,  1858. 

t  Methods  of  Study  in  Natural  History,  L.  Agassiz,  8th  ed.,  76,  1873. 


DEVELOPMENT  OF  THE  BRACHIOPODA  243 

tremata  comprises  the  Kampylopegmata  and  Helicopegmata 
of  the  same  author.  With  the  transfer  of  some  genera  in  his 
sub-orders,  they  may  be  properly  recognized  and  serve  further 
to  differentiate  the  class  into  comprehensive  groups. 

After  this  preliminary  discussion  the  four  groups  pro- 
posed can  be  denned  and  understood.  The  special  details 
with  full  illustration  and  demonstration  of  the  development 
and  affinities  in  each  group  are  left  for  future  consideration. 
At  present  it  is  aimed  to  give  only  the  general  results  which 
have  been  reached  through  the  study  of  individual  develop- 
ment (ontogeny)  among  various  species  representing  the 
families  of  nearly  the  entire  class.  Of  the  sixteen  families 
of  Brachiopoda  recognized  by  CEhlert  in  Fischer's  "Manuel 
de  Conchyliologie, "  fifteen  have  thus  been  studied  and  deter- 
mined. The  genera  marked  by  an  asterisk  have  been  exam- 
ined somewhat  in  detail.  The  others  have  been  investigated 
partly  from  adult  specimens,  and  from  the  published  descrip- 
tions of  the  genera. 

• 

ATREMATA. 

(a  priv.,  and  rprjfjia  perforation.) 
(Plate  XI,  figures  1-4.) 

Protegulum  semi-circular  or  semi-elliptical;  hinge-line 
straight  or  slightly  arcuate.  Growth  taking  place  mainly 
around  the  anterior  and  lateral  margins,  never  enclosing  or 
surrounding  the  pedicle,  which  in  all  stages  emerges  freely 
between  the  two  valves,  the  opening  being  more  or  less  shared 
by  both.  Valves  inarticulate. 

Including  the  genera  : 

Dignomia.  *Leptobolus.  Obolus. 

Dinobulus.  *Lingula.  *Iphidea. 
Elkania.                       Lingulasma.  Paterula. 

Glossina.  *Lingulops.  Rhynobolus. 

*Glottidia.  Monomer  ella.  Trimerella. 

Lakhmina.  Obolella. 


244  STUDIES  IN  EVOLUTION 

NEOTREMATA. 
(i>eo9  young,  and  rprjfjia  perforation.) 

(Plate  XI,  figures  5-12.) 

Protegulum  as  in  the  preceding  order  in  primitive  forms, 
becoming  more  circular  and  with  shorter  and  more  arcuate 
hinge  in  the  pedicle  valve  of  derived  types.  Growth  of  the 
dorsal  valve  tending  to  become  peripheral.  In  the  opposite 
valve  the  pedicle  more  or  less  surrounded  by  progressive 
neanic  growth  posterior  to  the  initial  hinge.  Pedicle  fissure 
remaining  open  in  primitive  mature  forms,  becoming  enclosed 
in  secondary  forms  during  neanic  stages,  and  in  derived 
types  enclosed  in  early  neanic  or  nepionic  stages.  Valves 
inarticulate. 
1  Including  the  genera : 

Anvistocrania.  *Disdnopsis.  Pholidops. 

Acrothele.  Helmersenia.  Pseudocrania. 

Acrotreta.  Kayserlingia.  *Roemerella. 

*Conotreta.  Lindstroemella.  *Schizambon. 

* Crania.  *Linnarssonia.  *  Schizobolus. 

*Oraniella.  Mesotreta.  *Scliizocrania. 

Craniscus.  *(Ehlertella.  Siphonotreta. 

*Discina.  *  Orbiculoidea.  *Trematis. 
*Distinisca. 

PROTREMATA. 

(Trpco  early,  and  rprj^a  perforation.) 

(Plate  XI,  figures  13-21.) 

Protegulum  of  the  dorsal  valve  as  in  the  Atremata.  In 
the  ventral  valve  it  has  become  modified  to  an  elliptical  or 
circular  form  with  arcuate  hinge.  Pedicle  enclosed  in  early 
nepionic  stages  by  a  pro-deltidium ;  posterior  covering  (deltid- 
ium)  retained  at  maturity,  or  resorbed  or  abraded  in  neanic 
stages,  so  that  the  pedicle  protrudes  between  the  two  valves. 
Valves  articulate. 


DEVELOPMENT  OF  THE  BRACHIOPODA 


245 


Including  the  genera : 

Ampliigenia. 
Aulosteges. 
Bactrynium. 
BiloUtes. 

Camarella  (group). 
Camarophoria. 
*Chonetes. 
Clitambonites. 
Conchidium. 
Davidsonella. 
Davidsonia. 
Daviesiella. 
Derbya. 
Enteletes. 
Eudesella. 
Hemipronties. 


Hipparionyx* 
*Lacazella. 
*Leptcena. 

Leptcenisca. 

Lyttonia. 

Meekella. 

Mimulus. 

Oldhamina. 
*0rthis  (group). 

Orthisina. 
*0rthothetes. 

Pentamerella. 

Platystrophia. 
*Plectambonites. 

Porambonites? 

Proboscidetta. 


*Productella. 
Productus. 

*  Rhipidomella. 
Schizopkoria. 
Sieberella. 
Streptis. 

*8treptorhynchus. 
Stricklandinia. 
Strophalosia. 

*  Stroplwodonta. 

*  Strophomena. 

*  Strophonella. 
ThecideUa. 

*  Thecidium. 
Thecidopsis. 
Triplecia. 


TELOTREMATA. 

(reXo?  last,  and  rp^^a  perforation.) 

(Plate  XI,  figures  22-28.) 

Protegulum  as  in  Atremata.  Pedicle-opening  shared  by 
both  valves  in  nepionic  stages,  usually  confined  to  one  valve 
in  later  stages,  and  becoming  more  or  less  limited  by  two 
deltidial  plates  in  ephebic  stages.  Arms  supported  by  cal- 
careous crura,  spirals,  or  loops.  Valves  articulate. 

Including  the  genera : 


Acanthotliyris . 

Ambocoe.Ua. 

Amphidina. 
*Athyris. 

*Atretia  (Cryptopora). 
*Atrypa. 

Bifida. 

Bouchardia. 

Centronella. 
*Cistella. 

Clorinda. 
*  Ccelospira. 


Ccenothyris. 

Cryptonella. 

Cyrtia. 

Cyrtina. 

Dayia. 

Dictyothyris. 

Dielasma. 

Dimerella. 

Disculina. 

Eatonia. 

Eudesia. 

Eumetria. 


Glassia. 

Grunewaldtia. 
*Hemithyris. 

Hindella. 

Ismenia. 

Karpinskya. 

Kayseria. 

Kingena. 

Koninckella. 
*  Koninckina. 
*J£raussina. 
*Laqueus. 


246  STUDIES  IN  EVOLUTION 

Leptocodia.  Nucleospira.  Stringoceplialus. 

Leiorhynclms.  Pentagonia.  Suessia. 

*Liothyrina.  Peregrinella.  Syringothyris. 

*Macandrevia.  Platydia.  *  Terebratella. 

Magas.  Rensselceria.  Terebratula. 

*Magellania.  Reticularia.  *  Terebratulina. 

*Martinia.  Eetzia.  Terebratuloidea. 

Martinopsis.  *  Rliynclionella.  Tliecospira. 

Megatliyris.  Rliynclionellina  Trematospira. 

Megalanteris.  Rhynclioporina.  Trigonosemus. 

*Megerlina.  Wiynchotrema.  *Tropidoleptus. 

Merista.  * Ehynchotreta.  Uncinulus. 

*Meristella.  *JSpirifer.  Uncites. 

*Meristina.  Spiriferina.  Zellania. 

*Muhlfeldtia.  Spirigerella.  *Zygospira. 


PART  II.     CLASSIFICATION  OF  THE  STAGES  OF  GROWTH 
AND  DECLINE* 

(PLATE  XII) 

A  BRIEF  review  of  the  known  embryology  of  the  Brachiop- 
oda  is  desirable,  in  order  to  account  for  some  of  the  differ- 
ences presented  by  adult  forms  in  the  several  divisions  of  the 
class.  This  knowledge  is  far  from  complete,  and  is  confined 
to  a  few  species,  but  much  of  interest  bearing  on  the  later 
development  of  the  organism  may  be  obtained. 

The  important  memoirs  •(•  of  Morse,18' 19  Kovalevsld,15  Lacaze- 
Duthiers,16  and  Shipley22  contain  nearly  all  that  is  known 
regarding  the  early  embryology  of  brachiopods.  The  genera 
included  in  the  works  of  these  authors  comprise  Cistella, 
Terebratulina,  Liothyrina,  and  Lacazella.  Later  larval  stages 
of  the  genus  G-lottidia  have  been  fully  described  by  Brooks.4 
Miiller,20  also,  has  given  a  description  and  figures  of  a  larval 
form  doubtfully  referred  to  Discinisca.  The  results  of  these 
observers  must  at  present  be  taken  without  reservation,  and 
are  thus  made  use  of  in  the  present  paper. 

*  Amer.  Jour.  Set.  (3),  XLIV,  133-155,  pi.  i,  1892. 

t  The  works  referred  to  by  numbers  are  cited  in  full  in  the  list  appended  to 
this  article. 


DEVELOPMENT  OF  THE  BRACHIOPODA  247 

Something  is  known,  therefore,  of  the  early  stages  in  each 
of  the  four  groups  or  orders  proposed  by  the  writer.2  The 
Atremata,  Neotremata,  arid  Protremata  are  represented  by  a 
single  genus  only  in  each;  Grlottidia,  Discinisca,  and  Laca- 
zella, respectively;  and  the  Telotremata,  by  Cistella,  Tere- 
bratulina,  and  Liottiyrina.  Were  Grlottidia  and  Discinisca  as 
well  known  as  Cistella,  Terebratulina,  and  Lacazella,  some 
comparisons  could  undoubtedly  be  made  which  would  en- 
lighten many  obscure  points  of  anatomy  and  morphology,  as 
well  as  give  clearer  insight  into  the  history  and  origin  of 
each  group. 

Cistella  and  Terebratulina  are  taken  as  standards  of  the 
embryological  development  on  account  of  the  completeness 
with  which  they  have  been  studied,  and  because  their  points 
of  difference  are  not  great.  Lacazella  shows  such  peculiar 
features  that  its  history  must  be  discussed  separately.  The 
nepionic  Grlottidia  and  Discinisca,  too,  present  characters 
which  evidently  had  an  early  history  somewhat  different 
from  Cistella  or  Terebratulina.  * 

In  taking  up  the  review  of  the  observed  stages  of  growth, 
an  attempt  will  be  made  to  fix  their  limitations.  To  this 
end  the  admirable  nomenclature  proposed  by  Hyatt 9>  10  is 
here  adopted,  as  it  is  more  convenient  and  of  wider  applica- 
tion and  significance  than  the  terms  heretofore  used.  Thus 
far  this  system  has  been  employed  principally  in  studies  relat- 
ing to  the  Mollusca,  and  its  application  to  the  Brachiopoda 
will  necessarily  require  some  illustration  and  explanation. 
In  the  preface  to  "  Genesis  of  the  Arietidse  "  Hyatt  has  pre- 
sented a  summary  of  the  theoretical  opinions  resulting  mainly 
from  his  studies  in  the  Cephalopoda.  It  is  believed  that 
nearly  the  same  ground  may  be  covered  in  the  Brachiopoda, 
and  thus  the  truth  of  these  deductions  will  receive  further 
evidence  from  another  class  of  organisms. 

Embryonic  Stages. 

The  true  embryonic  stages  are  classified  by  Hyatt  as  Pro- 
tembryo,  Mesembryo,  Metembryo,  Neoembryo,  and  Typembryo* 


248 


STUDIES  IN  EVOLUTION 


To  these  Jackson  n  has  added  the  PTiylembryo,  taking  it  from 
the  later  stages  of  the  Typembryo  to  represent  the  period 
when  the  animal  can  be  referred  definitely  to  the  class  to 
which  it  belongs. 

The  succeeding  stages  in  the  growth  of  the  animal  to 
maturity  are  termed  by  Hyatt  [and  emended  by  Buckman 
and  Bather]  nepionic  (young),  neanic  (adolescent),  and  ephebic 
(mature),  while  old-age  characters  are  called  gerontic.  The 
stages  are  further  divided  by  using  the  prefixes  ana,  meta, 
and  para;  as  anagerontic,  metagerontic,  and  paragerontic. 

The  application  of  this  nomenclature  of  the  stages  of 
growth  and  decline  to  the  Brachiopoda  is  shown  on  the  fol- 
lowing pages. 


85 


86 


87 


Cistella  neapolitana  Scacchi. 

FIGURE  85.  —  Protembryo;  unsegmented  ovum. 

FIGURE  86.  —  Protembryo ;  ovum  composed  of  two  spheres. 

FIGURE  87.  —  Mesembryo  ;  blastosphere. 

FIGURE  88.  —  Metembryo;  gastrula.     (Figures  85-88,  after  Shipley.) 

The  Protembryo,  as  in  other  groups  of  organisms,  includes 
the  ovum  and  its  segmented  stages  preceding  the  formation 
of  a  blastula  cavity.  Figures  85  and  86  show  protembryonic 
stages  of  Cistella.  The  eggs  are  spherical,  pyriform,  or  ovoid, 
and  the  segmentation  proceeds  in  a  regular  manner,  resulting 
in  a  blastosphere  composed  of  equal  parts. 

The  Mesembryo,  or  blastosphere  (figure  87),  has  been  ob- 
served in  Cistella,  Terebratulina,  and  Lacazella.  The  blas- 
tula cavity  is  small. 

The  Metembryo,  or  gastrula  stage  (figure  88),  is  developed 
from  the  blastosphere  in  two  ways :  (a)  by  embolic  in  vagina  - 
tion  in  Cistella  and  Terebratulina  (Kovalevski  and  Shipley), 
and  (5)  by  delamination  in  Lacazella  (Kovalevski).  At  the 
close  of  this  stage  the  archenteron  in  Cistella  is  trilobed, 


DEVELOPMENT  OF  THE  BRACHIOPODA 


249 


consisting  of  a  central  cavity,  or  mesenteron,  connecting  on 
each  side  with  the  body  cavity. 

The  Neoembryo,  represented  by  the  trochosphere  and  seg- 
mented, ciliated,  cephalula  stages,  has  been  more  fully  ob- 
served than  any  of  the  preceding.  The  first  advance  from 
the  completed  gastrula  is  in  the  separation  of  the  mesenteron 
from  the  body  cavity,  and  the  division  of  the  organism  into 
two  segments  or  lobes,  the  cephalic  and  caudal  (figure  89). 
Later  a  third  or  thoracic  segment  is  developed  and  carries 
four  bundles  of  stiff  barbed  setse  (figure  90).  The  cephalic 
and  caudal  lobes  are  densely  ciliated.  During  the  subse- 
quent cephalula  period  two  eyes,  then  two  others,  appear  in 
Cistella,  and  at  the  same  time  the  dorsal  and  ventral  sides 
of  the  thoracic  segment  become  extended  over  the  caudal, 
and  are  progressively  defined  as  two  lobes  (figures  89-93, 
108,  109). 

89  90  91  92 


Cistella  neapolitana  Scacchi. 

FIGURE  89.  —  Neoembryo ;  embryo  of  two  segments. 

FIGURE  90.  —  Neoembryo ;  cephalula ;  ventral  side ;  showing  cephalic,  thoracic, 
and  caudal  segments,  eye-spots,  and  bundles  of  seta3.  (Figures  89  and  90,  after 
Kovalevski.) 

FIGURE  91.  —  Neoembryo;  lateral  view  of  completed  cephalula  stage;  show- 
ing extent  of  dorsal  (d)  and  ventral  (v)  mantle  lobes,  and  umbrella-like  cephalic 
segment. 

FIGURE  92.  —  Neoembryo;  same  stage;  ventral  view.  (Figures  91  and  92, 
after  Shipley.) 

Terebratulina  has  a  tuft  of  bristles  on  the  top  of  the 
cephalic  segment.  In  Lacazella  the  bundles  of  set83  are 
absent,  and  the  head  is  more  distinctly  differentiated  from 
the  anterior  segment  than  in  Cistella.  The  closing  cepha- 
lula stage  in  Cistella  has  an  umbrella-like  expansion  of  the 


250 


STUDIES  IN  EVOLUTION 


cephalic  border,  and  the  organism  becomes  a  free-swimming 
larva  (figures  91-93). 

Larval  Stages. 

The  Typembryo  is  the  larval  stage  at  which  some  distinc- 
tive features  make  their  appearance,  but  before  the  special 
characters  of  the  class  are  to  be  found  (figure  94).  It  is 
analogous  to  the  molluscan  embryo  in  which  a  shell  gland 
and  plate-like  initial  shell  are  developed.  There  is,  however, 
no  homology  of  parts  or  organs  between  the  t}~pembryonic 
mollusk  and  brachiopod. 


Cistella  neapolitana  Scacchi. 

FIGURE  93.  — Neoembryo  ;  completed  cephalula  stage. 

FIGURE  94.  —  Typembryo;  transformed  larva  resulting  from  folding  up- 
ward of  mantle  lobes  over  cephalic  segment,  ad,  muscles  from  bundles  of  setse 
to  sides  of  body  cavity  ;  di,  muscles  from  dorsal  to  ventral  sides  of  body ;  vp, 
muscles  from  ventral  side  of  body  to  caudal  segment  or  pedicle.  (Figures  93 
and  94,  after  Kovalevski.) 

In  Cistella  and  Terebratulina  the  development  of  the  typ- 
embryo  has  been  observed,  and  consists  of  the  folding 
upward  of  the  lobes  which  have  been  developed  from  the 
thoracic  segment  to  form  the  mantle,  so  that  they  gradually 


DEVELOPMENT  OF  THE  BRACHIOPODA  251 

enclose  the  anterior  end  (figures  108-111).  The  surfaces  of 
the  mantle  which  were  exterior  in  the  cephalula  have  now 
become  inner  and  the  bundles  of  setse  have  revolved  180°, 
changing  their  direction  from  posterior  to  anterior.  This 
leaves  the  lower  part  of  the  thoracic,  and  the  whole  of  the 
caudal,  segment  exposed.  The  outer  surface  of  the  mantle  is 
invested  with  a  hard  integument,  which,  upon  completion 
and  before  the  growth  of  the  true  shell,  forms  the  protegu- 
lum.  The  pedicle  at  this  stage  is  also  defined,  being  a  modi- 
fication of  the  caudal  segment.  It  may  serve  to  attach  the 
larva  to  foreign  objects,  as  in  Cistella  (figure  94)  and  Tere- 
bratulina,  or  it  may  remain  undeveloped  for  a  time,  as  in 
Grlottidia  and  Discinisca.  A  rudimentary  digestive  tract  is 
present. 

The  body  muscles  which  have  been  developed  thus  far 
consist  of  four  distinct  pairs.  Two  pairs  lie  close  to  the 
sides  of  the  body  cavity,  and  extend  to  the  points  of  inser- 
tion of  the  bundles  of  bristles  (figure  94,  ad).  They  become 
after  transformation  the  four  adductor  muscles  of  the  valves. 
The  third  pair  extends  from  the  ventral  side  of  the  body  to 
the  caudal  segment,  and  is  converted  into  the  ventral  pedicle 
muscles  (figures  94,  99,  100,  vp).  The  fourth  pair  is  situated 
posterior  to  the  digestive  tract,  and  extends  from  the  dorsal 
to  the  ventral  wall  of  the  body  (figure  94,  di).  They  form 
the  divaricator  muscles  in  the  mature  brachiopod  (figure  100, 
di),  and  are  divided  into  or  duplicated  by  a  pair  of  dorsal  and 
a  pair  of  ventral  divaricators.  There  is  also  a  pair  of  dorsal 
pedicle  muscles  in  the  larva  of  LiotJiyrina  and  Terebratulina. 

The  folding  upward  of  the  mantle  lobes  forms  the  first 
hinge-line  of  the  future  valves  (hi,  figures  110,  111).  Thus 
its  origin  is  not,  as  in  pelecypods,  a  line  produced  by  the 
bending  of  a  single  plate  (Jackson),  but  is  the  line  along 
which  the  two  mantle  lobes  are  bent  against  the  body. 
Between  them  projects  posteriorly  nearly  half  the  body  of 
the  animal,  and  the  whole  opening  corresponds  to  the  pedicle- 
opening  of  later  stages  of  growth.  The  hinge  of  brachiopods, 


252  STUDIES  IN  EVOLUTION 

therefore,  is  not  primarily  a  line  of  articulation  of  the  valves, 
but  the  limiting  borders  between  the  body  and  the  attached 
edges  of  the  mantle.  Secondarily,  and  during  later  growth, 
the  extension  of  the  valves  along  a  line  of  apposition  forms  a 
true  hinge-line. 

The  first  points  of  contact  of  the  valves  to  form  the  true 
hinge  lie  adjacent  to  the  right  and  left  sides  of  the  body  of 
the  animal,  at  the  cardinal  extremities  (figure  99,  t).  Here 
naturally  the  first  hinge-teeth  are  formed,  and  their  position 
corresponds  to  that  in  adult  individuals;  namely,  on  each 
side  of  the  cardinal  opening.  The  enlarging  of  the  cardinal 
opening  by  shell  growth  results  in  the  gradual  divergence 
or  separation  of  the  teeth,  as  in  Terebratulina.  In  species 
with  extended  hinge-lines,  as  in  many  forms  of  Spirifer, 
Orthis,  and  Strophomena,  the  teeth  still  lie  in  their  original 
position  on  each  side  of  the  cardinal  opening,  and  the  elon- 
gation of  the  hinge  has  come  not  only  from  the  enlargement 
of  the  opening  by  growth,  but  by  additions  at  the  hinge 
extremities,  so  that  the  teeth  are  situated  on  each  side  of  the 
central  area,  below  the  beak,  and  not  at  the  cardinal  angles. 
The  young  of  these  genera,  however,  all  have  the  hinge-teeth 
at  the  extremities  of  the  hinge,  as  the  cardinal  opening  then 
occupies  the  whole  posterior  area  of  the  shell. 

Adult  specimens  of  Kutorgina  (K.  cingulata  Billings)  have 
a  deltidium  as  in  Strophomena.  The  cardinal  opening  in- 
cluding the  deltidium  occupies  the  whole  posterior  end  of 
the  shell,  and  according  to  a  statement  made  to  the  writer 
by  Mr.  Charles  Schuchert,  there  are  rudimentary  teeth  at 
the  cardinal  extremities.  Therefore  this  genus  represents  a 
nepionic  condition  of  later  forms,  and,  on  account  of  these 
and  other  characters,  it  is  believed  to  be  related  to  Orthisina 
and  Strophomena,  of  which  it  is  the  ancestral  type.  It  con- 
sequently belongs  to  the  articulate  brachiopods. 

The  embryonic  stages  up  to  this  point  have  frequently 
been  compared  to  similar  stages  in  other  organisms,  especially 
in  the  Annelida  and  Polyzoa.  Without  repeating  these  com- 


DEVELOPMENT  OF  THE  BRACHIOPODA 


253 


parisons,  which  may  be  consulted  elsewhere,4'  12» 15>  16~19'  21 
attention  is  called  to  the  similarity  of  development  of  the 
brachiopod  typembryo  to  the  larval  stages  of  Spirorbis. 
There  are,  however,  important  structural  differences.  An 
article  by  J.  W.  Fewkes,  "  On  the  Larval  Forms  of  /Spirorbis 
borealis  Daudin,"  7  contains  a  nearly  complete  and  very  inter- 
esting account  of  the  development  of  this  chsetopod.  There 
is  a  striking  resemblance  in  the  characters  of  the  cephalula 


98 


96 


Spirorbis  borealis  Daudin. 

FIGURE  95.  —  Cephalula,  developing  lobe  from  the  body  (col). 

FIGURE  96.  —  More  advanced  stage. 

FIGURE  97.  —  Larval  form  before  transformation ;  showing  posteriorly  directed 
expansion  (col)  from  thoracic  segment. 

FIGURE  98.  —  Transformed  Spirorbis ;  showing  folding  upward  of  collar 
partially  enclosing  head.  (Figures  95-98,  after  Fewkes.) 

stages  in  both  organisms,  as  may  be  seen  on  comparison 
(figures  95  and  96).  Spirorbis  develops  a  posteriorly  directed 
extension  from  the  middle  segment,  called  a  collar,  which  in 
later  stages  is  reflexed  anteriorly  so  as  to  cover  more  or  less 
the  cephalic  portion,  thus  agreeing  with  the  growth  and 
change  in  position  of  the  mantle  in  Cistella.  The  ventral 
lobe  is  also  the  larger  in  both.  Many  other  comparisons 
and  homologies  have  been  made  by  Morse,19  and  the  one  here 
described  is  even  more  marked  than  his  reference  to  the 


254 


STUDIES  IN  EVOLUTION 


lobation  of  the  cephalic  collar  in  Sabella.  Four  figures  are 
introduced  illustrating  the  principal  changes  in  Spirorlis. 
They  may  be  compared  with  the  development  of  Cistella 
shown  in  figures  90—94. 

It  is  not  intended  by  this  to  indicate  a  close  relationship 
with  the  chsetopods,  for  the  writer  is  inclined  to  accept  the 
opinion  of  Joubin,12  that  the  brachiopods  constitute  a  distinct 
and  independent  class. 

The  Phylembryo  (figure  99)  differs  from  the  typembryo  in 
(a)  the  completion  of  the  embryonic  shell,  or  protegulum; 
(6)  the  first  appearance  of  the  tentacular  lobes  of  the  lopho- 


vf 


Cistella  neapolitana  Scacchi. 

FIGURE  99.  —  Phylembryo;  brachiopod;  showing  shell  (protegulum),  begin- 
ning of  tentacles  of  lophophore  (/),  obsolescence  of  eye-spots,  and  formation  of 
oesophagus  ;  t,  hinge-teeth  ;  vp,  ventral  pedicle  muscles. 

FIGURE  100.  —  Nepionic  brachiopod  ;  showing  distinct  tentacles  of  lophophore, 
mouth,  and  stomach,  and  transformation  of  muscles  from  typembryo  (figure  94). 
ad,  adductors;  di,  divaricators ;  vp,  ventral  pedicle  muscles.  (Figures  99  and 
100,  after  Kovalevski.) 

phore,  or  arms;  (V)  the  usual  dehiscence  of  the  four  bundles 
of  setse ;  (d)  the  obsolescence  of  the  eyes ;  (e)  the  definition 
of  the  oesophagus  and  stomach,  and  (/)  the  agreement  of 
the  muscular  system  with  that  in  adult  forms.  These  fea- 
tures, with  the  pedicle  which  appeared  in  a  preceding  stage, 
represent  the  brachiopod  phylum,  and  are  properly  referred 
to  the  phylembryonic  period  of  Jackson.  Although  the 
molluscan  stage  called  the  prodissoconch  in  pelecypods,  the 


DEVELOPMENT  OF   THE  BRACHIOPODA  255 

protoconch  in  cephalopods  and  gastropods,  and  the  periconch 
in  scaphopods,  represents  the  completed  phylembryo  of  these 
groups,  as  the  protegulum  represents  a  like  period  in  the 
developing  brachiopod,  yet  there  is  no  homology  of  distinc- 
tive organs. 

The  mantle  of  mollusks  is  first  formed  on  the  posterior 
dorsal  side,  and  is  in  the  shape  of  a  disk,  which  gradually 
envelops  the  animal  to  a  greater  or  less  extent,  and  may 
become  distinctly  lobed.  As  has  been  shown,  this  organ  in 
the  brachiopods  develops  simultaneously  from  the  dorsal  and 
ventral  sides  of  the  thoracic  segment  of  the  cephalula,  and  is 
primarily  bilobed. 

The  initial  shell  of  brachiopods  is  not  produced  from  a  dis- 
tinct shell  gland,  as  in  the  Mollusca,  but  is  an  integument 
of  the  surface  of  the  mantle  lobes,  and  intimately  connected 
with  them.  The  position  of  the  valves  is  dorsal  and  ventral. 
The  pedicle  has  no  organic  similarity  with  either  a  foot  or  a 
byssus. 

The  mouth  of  mollusks  (and  annelids)  is  formed  below  the 
base  of  the  cephalic  lobe  of  the  cephalula,  and  may  be  the 
blastopore,  while  in  the  brachiopods  it  is  near  the  anterior 
pole  within  the  cephalic  segment.  Notwithstanding  these 
differences,  so  many  parts  are  functional  equivalents  that 
their  growth  and  development  may  be  discussed  and  inter- 
preted in  the  same  terms. 

Before  passing  to  later  stages  of  growth  which  become 
more  and  more  divergent  from  a  common  simple  type,  some 
points  previously  omitted,  relating  to  Thecidium  (Lacazella), 
Lingula  (Grlottidia),  and  Ditcinitca,  should  be  here  noted. 
As  Lacazella  is  a  form  in  which  the  ventral  valve  in  the 
neanic  and  ephebic  stages  is  cemented  to  foreign  objects  by 
calcareous  fixation,  it  bears  about  the  same  relation  to  other 
brachiopods  that  Ostrea  bears  to  Avicula,  among  the  pelecy- 
pods,  and  a  corresponding  early  absence  or  modification  of 
many  features  present  in  adult  individuals  should  be  looked 
for.  From  what  is  known  of  the  geological  history  of  The- 
cidium, and  if  the  interpretations  of  its  phylogeny  by  the 


256  STUDIES  IN  EVOLUTION 

writer  are  correct,  it  is  derived  from  an  ancestry  which  had  a 
similar  condition  of  fixation  as  early  as  the  Upper  Silurian. 
Thecidium  is  apparently  not  a  terebratuloid  genus.  Its  struc- 
tural affinities  are  evidently  with  the  strophomenoids,  espe- 
cially such  forms  as  Plectambonites,  Leptoenisca,  etc.  Briefly 
the  reasons  for  this  statement  are  (a)  the  presence  of  a  del- 
tidium  of  one  plate ;  (6)  the  absence  of  a  true  loop  supporting 
the  arms  (the  internal  calcification  or  spiculization  is  confined 
wholly  to  the  mantle,  and  does  not  extend  to  the  arms  16) ; 
(c)  a  concave  plate  in  the  cavity  of  the  ventral  beak,  bearing 
the  divaricator  muscles ;  (d)  the  attached  ventral  valve,  and 
(e)  the  cardinal  processes  in  the  dorsal  valve.*  The  first 
character  is  of  prime  importance,  because  all  the  strophom- 
enoids and  none  of  the  terebratuloids  have  a  deltidium  of 
one  plate. 

It  would  appear,  therefore,  that  the  early,  free-swimming, 
larval  state,  and  the  later  pediculate  stage  have  become  lost 
by  acceleration,  thus  accounting  for  the  very  unequal  develop- 
ment of  the  mantle  lobes  in  the  cephalula  stage,  and  the  non- 
active  and  early  sedentary  larvae  as  described  by  Kovalevski 
and  Lacaze-Duthiers. 

The  young  Lingula  (G-lottidia)  described  by  Brooks,  and 
the  Discinisca  by  M  tiller,20  both  representing  the  phylem- 
bryonic  stage,  were  active  and  free-swimming  animals,  with 
rudimentary  pedicles.  Terebratulina  becomes  attached  or 
rests  on  the  caudal  segment  during  the  cephalula  stage 
(Morse),  while  at  the  end  of  this  period  in  Cistella  (Kova- 
levski and  Shipley)  there  is  an  active,  swimming,  ciliated 
organism,  which  later  attaches  itself  by  the  pedicle  in  the 
typembryonic  period. 

From  the  facts  that  young  individuals  of  Paleozoic  species 
belonging  to  such  genera  as  Zygospira,  Spirifer,  Orthis, 
Rhynchonella,  and  Scenidium,  have  been  observed  by  the 

*  Dall  in  1870  (Amer.  Jour.  Conchology)  made  a  clear  statement  of  the 
characters  of  Thecidium  and  of  many  of  its  radical  points  of  difference  with  the 
Terebratulidae,  showing  that  it  was  entitled  to  rank  as  the  type  of  a  distinct 
family. 


DEVELOPMENT  OF  THE  BRACHIOPODA  257 

writer  to  retain  their  original  relations  to  the  objects  of  sup- 
port, and  that  casts  of  the  pedicles  of  fossil  Lingulse  and 
Eichwaldia  have  been  described  (Davidson,5  Walcott23),  it 
cannot  be  assumed  that  the  free-swimming  condition  was 
ever  present  in  neanic  or  ephebic  individuals.  Evidently  it 
has  always  been  a  larval  character. 

Origin  of  the  Deltidium  and  Deltidial  Plates. 

The  origin  and  significance  of  the  deltidium  *  (=  "  pseudo- 
del  tidium  ")  are  made  apparent  in  the  development  of  The- 
cidium,  and  it  may  be  well  in  this  place  to  make  a  few 
observations  on  the  genesis  of  this  important  character,  and 
its  relations  to  the  deltidial  plates  of  other  genera,  as 
Rhynchonella  and  Terebratula.  It  has  been  already  noted 
(Part  1),  that  the  deltidium  in  all  species  possessing  it  (the 
Protremata)  is  an  embryological,  or  nepionic  feature,  which 
may  or  may  not  continue  to  the  ephebic  period;  while  the 
deltidial  plates  in  other  brachiopods  (the  Telotremata)  appear 
later  during  the  neanic  and  ephebic  periods,  or  may  never 
be  developed.  The  detailed  researches  of  Kovalevski  on 
Cistella  and  Thecidium,  together  with  other  observations  now 
first  made,  furnish  data  for  a  clear  understanding  of  these 
differences.  J 

Figure  102  represents  a  dorso-ventral  section  of  a  ripe 
cephalula  just  before  the  transformation,  and  shows  the  un- 

*  The  single  plate  or  covering  to  the  triangular  opening  beneath  the  ventral 
beak  should  be  termed  the  deltidium,  as  it  was  thus  extensively  used  by  David- 
son. When  it  consists  of  two  plates,  they  may  be  called  deltidial  plates.  These 
names  have  been  loosely  used.  In  Part  I  of  this  paper  the  deltidium  proper  is 
referred  to  as  pedicle  covering,  pedicle-sheath,  and  pseudo-deltidium.  Hall  and 
Clarke  have  proposed  to  call  the  triangular  opening  in  the  beaks  of  brachiopods 
the  delthyrium,  and  the  concave  plate  in  the  ventral  beak  of  Pentamerus, 
Orthisina,  etc.,  they  have  termed  the  spondylium.  There  yet  remains  a  term  for 
the  convex  plate  covering  the  opening  below  the  beak  of  the  dorsal  valve,  and 
resembling  the  deltidium  of  the  opposite  valve.  For  this  feature  the  name 
chilidium  (x^Aos)  is  here  proposed. 

J  Kovalevski 15  For  Thecidium  consult  the  explanation  of  pi.  iv,  figs.  15-26  ; 
for  Cistella,  pi.  i,  figs.  13-15;  pi.  ii,  figs.  17,  19-21. 

17 


258 


STUDIES  IN  EVOLUTION 


equal  lobes  of  the  mantle,  v  being  the  ventral  lobe,  and  d 
the  dorsal;  h  is  the  head,  and^?  the  caudal  segment  develop- 
ing into  a  pedicle.  A  deposit  of  integument  representing 


Thecidium  (Lacazelld)  mediterraneum  Risso. 

FIGURE  101.  —  Cephalula;  dorsal  side,  ds,  dorsal  shell  plate;  h,  head. 
(After  Kovalevski.) 

FIGURE  102. — Dorso-ventral  longitudinal  section  of  cephalula  of  about  same 
age  as  preceding,  h,  head ;  d,  dorsal  mantle  lobe ;  v,  ventral  mantle  lobe ; 
ds,  beginning  of  dorsal  valve;  del,  shell  plate  forming  on  dorsal  side  of  body; 
p,  pedicle.  (After  Kovalevski.) 

FIGURE  103. — Typembryo;  larva  transformed  from  folding  upward  of 
mantle  lobes,  h,  head ;  ds,  dorsal  valve ;  hi,  hinge-line  of  dorsal  valve ;  del, 
shell  plate  on  body  and  pedicle  posterior  to  hinge-line  of  dorsal  valve.  (After 
Kovalevski.) 

FIGURE  104.— Dorso-ventral  longitudinal  section  of  preceding.  References 
as  in  figure  103.  vs,  ventral  valve;  p,  pedicle. 

FIGURE  105.  —  Profile  view  of  neanic  Leptcena  rhomboidalis.  The  features  of 
the  shell  are  placed  and  lettered  as  in  figure  104.  ds,  dorsal  valve;  hi,  hinge- 
line  ;  del,  deltidium ;  p,  pedicle-opening ;  vs,  ventral  valve. 

FIGURE  106. — Adult  Thecidium  (LacazeUa)  mediterraneum;  dorsal  side; 
showing  ventral  area  and  deltidium. 

FIGURE  107.  —  Profile  of  same.    References  as  in  figures  104  and  105. 

the  shell  has  formed  on  the  inner  side  of  the  dorsal  mantle 
lobe  (c?s),  and  also  on  the  adjacent  dorsal  side  of  the  body 
lobe  (deT).  A  larva  somewhat  more  advanced  is  represented 
in  figure  101,  as  viewed  from  the  dorsal  side.  The  mantle 


DEVELOPMENT  OF  THE  BRACHIOPODA  259 

lobe  is  still  directed  posteriorly,  as  in  the  preceding  figure, 
and  the  underlying  shell  plate  is  shown  at  ds.  In  the  process 
of  transformation  (figures  103,  104),  the  mantle  lobe  is 
turned  forward  in  the  usual  manner,  bringing  the  shell  on 
the  outside  of  the  animal,  so  that  both  dorsal  plates  are  now 
exposed,  ds  being  the  dorsal  valve,  and  del  the  shell  devel- 
oped on  the  dorsal  side  of  the  walls  of  the  body  and  caudal 
segments.  As  this  plate  (del)  is  below  or  posterior  to  the 
hinge-line  (^Z),  and  extends  down  over  the  pedicle,  it  is 
evidently  the  beginning  of  the  deltidium.  At  the  same  time 
there  is  an  extension  of  the  edges  of  the  mantle  and  pedicle 
on  the  ventral,  or  lower,  side  and  shelly  matter  is  deposited, 
forming  the  ventral  valve  (vs,  figure  104).  At  this  stage 
the  hinge-line  (figures  103,  104,  hi)  is  the  line  between  the 
dorsal  mantle  shell  (ds)  and  the  dorsal  body  shell  plate  (del). 
The  beak  of  the  ventral  valve  is  separated  from  the  dorsal 
beak  by  the  pedicle  and  the  shell  covering  to  the  pedicle  and 
body  lobe,  or  the  deltidium.  The  valves  afterward  meet  at 
their  peripheries ;  the  hinge  is  extended  beyond  the  deltidium, 
forming  the  true  hinge  of  articulate  brachiopods.  As  there 
is  no  motion  between  the  ventral  valve  and  the  deltidium,  the 
two  become  ankylosed.  Figures  106  and  107,  showing  an 
adult  Thecidium,  are  lettered  in  the  same  manner  as  the  pre- 
ceding, and  express  the  same  relation  of  parts. 

The  deltidium  is  not,  therefore,  primarily,  on  account  of 
its  manner  of  origin,  an  integral  part  of  the  ventral  valve, 
but  is  a  shell  growth  from  the  dorsal  side  of  the  body,  which 
afterward  becomes  attached  to  the  ventral  valve,  and  is  then 
considered  as  belonging  to  it. 

The  further  growth  of  the  deltidium  around  the  body  and 
pedicle,  and  its  consequent  extension  into  the  cavity  of  the 
ventral  umbo,  may  explain  the  origin  of  the  spondylium. 

Kovalevski 15  believed  the  ventral  valve  in  Thecidium  was 
secreted  by  the  expanded  edges  of  the  pedicle  and  the  body 
walls ;  whether  or  not  this  is  so  does  not  affect  the  interpre- 
tation of  the  origin  of  the  deltidium.  From  the  observations 
of  Lacaze-Duthiers,16  it  seems,  however,  as  though  the  ven- 


260  STUDIES  IN  EVOLUTION 

tral  mantle  lobe  must  have  formed  the  shell  in  the  usual 
way.  This  appears  all  the  more  probable  from  the  fact  that 
the  lower  or  ventral  valve  is  punctate,  and,  so  far  as  known, 
the  mantle  contains  all  the  csecal  prolongations,  which  alone 
could  produce  the  punctate  structure.  Careful  microscopic 
examination  has  failed  to  detect  punctse  in  the  deltidia  of 
Thecidium,  Strophomena,  Leptcena,  and  other  punctate  genera 
belonging  to  the  Protremata. 

It  is  true  that  Aulosteges  has  spines  on  the  deltidium,  but 
spines  even  when  tubular  are  not  equivalent  to  punctse, 
as  shown  in  Producing  Strophalosia,  and  some  species  of 
Spirifer.  Aulosteges  is  a  gerontic  genus,  which  has  become 
excessively  spinose,  and  has  also  reverted  to  ancestral  char- 
acters in  its  high  hinge-area  and  conspicuous  deltidium.  It 
is  well  known  that  even  the  spires  of  Spiriferina  and  the  loop 
of  Macandrevia  are  spinose. 

Turning  now  to  Cistella  as  a  representative  of  the  Telo- 
tremata,  a  different  process  obtains. 

Figure  108  represents  the  fully  developed,  free-swimming 
cephalula  of  Cistella,  and  shows  the  extent  of  the  folds  of 
the  mantle  and  their  posterior  direction.  Figure  109  repre- 
sents the  same  in  section.  The  inner  sides  of  the  mantle 
lobes  are  to  form  the  future  valves,  the  dorsal  (ds\  and  the 
ventral  (vs) .  The  transformed  larva  or  typembryo  is  repre- 
sented in  figure  110  and  in  section  in  figure  111.  It  is  seen 
that  the  transformation  consists  in  the  folding  forward  of 
the  mantle  lobes  over  the  head  segment  (A).  Now  the  shell- 
secreting  layers  of  the  mantle  are  exterior,  and  the  two 
valves  begin  to  form,  the  dorsal  shell  (c?s),  and  the  ventral 
(vs).  The  pedicle  and  posterior  portion  of  the  body  come  out 
freely  between  the  valves  and  mantle  lobes  and  limit  the 
hinge-areas  of  both  (hi  and  M). 

The  further  process  of  growth  increases  the  distance  be- 
tween the  initial  dorsal  and  ventral  hinges,  for  while  the 
original  dorsal  beak  is  usually  maintained  at  the  hinge-line, 
the  ventral  beak  is  progressively  removed  and  the  ventral 
hinge  travels  from  its  first  position  at  the  beak,  along  the 


DEVELOPMENT  OF  THE  BRACHIOPODA 


261 


edges  of  the  umbo,  leaving  an  open  triangular  area  or  del- 
thyrium  in  the  ventral  valve  occupied  by  the  pedicle.     This 


108 


Cistella  neapolitana  Scacchi. 

FIGURE  108.  —  Lateral  view  of  completed  cephalula  stage,  h,  head  ;  d,  dorsal 
lobe  of  mantle;  v,  ventral  lobe;  p,  pedicle.  (After  Shipley.) 

FIGURE  109.  —  Dorso-ventral  longitudinal  section  of  same  :  showing  posteri- 
orly extended  mantle  lobes.  A,  head ;  ds  and  vs,  inner  surfaces  of  mantle  lobes 
which  are  to  form  dorsal  and  ventral  valves.  (After  Shipley.) 

FIGURE  110.  —  Typembryo;  dorsal  view  of  larva  after  transformation,  h, 
head;  ds,  dorsal  valve;  hi,  hinge-line  of  dorsal  valve;  p,  pedicle.  (After 
Kovalevski.) 

FIGURE  111.  —  Dorso-ventral  longitudinal  section  based  on  preceding; 
showing  mantle  lobes  directed  forward,  bringing  interior  shell-secreting  surfaces, 
ds  and  vs  of  figure  109,  on  the  exterior,  h,  head;  ds,  dorsal  valve;  hi,  dorsal 
hinge  ;  vs,  ventral  valve  ;  hi',  ventral  hinge  ;  p,  pedicle. 

FIGURE  112.  —  Dorsal  view  of  early  nepionic  shell;  showing  large  posterior 
opening  between  valves.  (After  Kovalevski.) 

FIGURE  113.  —  Profile  of  same,  ds,  dorsal  valve;  vs,  ventral  valve;  p, 
pedicle. 

condition  represents  the  extent  of  the  development  of  these 
parts  in  Meristina  rectirostris  Hall  or  Gwynia  capsula  Jef- 


262 


STUDIES  IN  EVOLUTION 


freys,  which  lack  deltidial  plates  in  the  adult  shell.  The 
young  of  other  telotremate  species,  as  Magellania  flavescens 
or  Terebratulina  septentrionalis,  agree  in  the  same  respect. 


114 


119 


115 


•-Id 


118 


120 


116 


FIGURE  114.  — Delthyrium  of  young  Rhynchonella,  without  deltidial  plates. 

FIGURE  115.  —  The  same  at  a  later  stage,  with  two  triangular  deltidial  plates. 

FIGURE  116. — The  same  after  completed  growth;  showing  joining  of 
deltidial  plates,  and  limitation  of  pedicle-opening  to  ventral  beak. 

FIGUBE  117.  —  Dorsal  view  of  Magellania  Jlavescens ;  showing  completed 
deltidial  plates,  del. 

FIGURE  118.  —  The  same;  profile,  ds,  dorsal  valve;  vs,  ventral  valve;  p, 
pedicle. 

FIGURE  119.  —  Dorsal  view  of  umbonal  portion  of  adult  Terebratulina 
septentrionalis,  with  shell  removed  by  acid;  showing  slight  secondary  exten- 
sion of  ventral  mantle  around  pedicle  (consequently  small  deltidial  plates  are 
secreted  in  this  species).  Mantle  areas  secreting  deltidial  plates  are  shaded. 

FIGURE  120.  —  Dorsal  view  of  umbonal  portion  of  Magellania  Jlavescens,  with 
the  shell  removed  by  acid  ;  showing  the  complete  envelopment  of  base  of  pedicle 
by  secondary  expansions  from  ventral  mantle,  and  consequent  production  of 
deltidial  plates  filling  delthyrium  except  at  pedicle-opening.  See  figure  117. 

An  examination  of  .the  animal  at  this  stage  shows  that  the 
mantle  lobes  line  only  the  interior  of  the  valves  proper. 
The  exposed  edges  of  the  mantle  are  around  the  peripheries 
of  the  valves  and  also  that  portion  of  the  ventral  mantle 


DEVELOPMENT  OF  THE  BRACHIOPODA  263 

border  limiting  the  deltidial  opening  and  passing  along  the 
sides  of  the  pedicle  at  its  base.  The  ventral  mantle  grad- 
ually extends  from  each  side  as  two  prolongations  partially 
covering  the  opening  and  enveloping  the  proximal  portion  of 
the  pedicle.  As  this  is  an  extension  of  the  shell-secreting 
surface  of  the  mantle,  there  naturally  results  the  formation 
of  two  plates  within  the  deltidial  area.  Their  structure  is 
commonly  punctate  whenever  the  valves  are  punctate. 

These  outgrowths  or  extensions  of  the  mantle  into  the  del- 
tidial area  finally  touch  and  coalesce  until,  as  in  M.  flavescens, 
the  pedicle  emerges  through  an  opening  in  the  ventral  mantle, 
and  pari  passu  the  deltidial  plates  unite  and  limit  the  pedicle- 
opening  to  the  beak  of  the  ventral  valve.  The  latter  process 
has  been  carefully  described  by  Deslongchamps,6  Clarke,  and 
the  writer,3  and  need  not  be  dwelt  on  here.  Figures  119  and 
120  of  the  beaks  of  T.  septentrionalis  and  M.  flavescens  with 
the  shell  removed  show  the  relations  of  the  ventral  mantle 
to  the  pedicle,  and  the  portions  which  secrete  the  deltidial 
plates.  ,- 

The  deltidium  and  delthyrium  are  often  simulated  in  the 
growth  of  the  dorsal  valve  in  genera  having  a  high  cardinal 
area  in  this  valve.  Orthis,  Leptcena,  Clitambonites,  Spirifer, 
and  Strioklandinia  may  be  cited  as  examples.  They  cannot 
be  properly  correlated  with  similar  parts  in  the  ventral  valve, 
for  their  origin  is  quite  different.  Primarily,  a  deltidial 
opening  is  for  the  extrusion  of  the  pedicle,  and  this  belongs 
properly  to  the  ventral  valve.  The  dorsal  fissure  is  the  space 
between  the  diverging  teeth  sockets,  and  may  be  filled  by  the 
cardinal  process,  as  in  Leptcena  and  Orthis,  or  it  may  have 
in  addition  a  convex  plate  or  chilidium  covering  it,  as  in 
Clitambonites.  In  Spirifer  and  Stricklandinia  the  opening 
remains  unclosed. 

The  true  deltidial  plates  are  formed  on  the  side  of  the 
pedicle  adjacent  to  the  hjnge  by  extensions  of  the  ventral 
mantle  lobe,  and  begin  as  two  plates.  They  are  likewise 
expressive  of  maturity,  and  are  of  secondary  development, 
while  the  deltidium  begins  as  a  single  plate  in  the  median 


264  STUDIES  IN  EVOLUTION 

line,  and   is   eminently  a   primitive  character  in   the   Pro- 
tremata. 

From  present  knowledge  of  the  group  it  is  difficult  to  offer 
an  explanation  for  the  presence  of  an  anal  opening  in  the 
Inarticulata  and  its  absence  in  the  recent  Artie  ulata,  as  the 
solution  of  the  question  depends  upon  whether  the  class  is  to 
be  considered  as  progressive  or  degraded.  The  dorsal  beaks 
of  Amphigenia.)  Athyris,  Cleiothyris,  Atrypa,  and  RJiyncho- 
nella  are  usually  notched  or  perforate.  The  perforation 
comes  from  the  union  of  the  crural  plates  above  the  floor 
of  the  beak  leaving  a  passage  through  to  the  apex.  A 
similar  opening  occurs  between  the  cardinal  processes  in 
Strophomena,  Stropheodonta,  and  allied  genera,  and  the  chi- 
lidium  may  also  be  furrowed,  as  in  Leptcena  rhomboidalis. 
This  character  is  evidently  in  no  way  connected  with  the 
pedicle-opening,  but  points  to  the  existence,  in  the  early  artic- 
ulate genera,  of  an  anal  opening  dorsal  to  the  axial  line,  as  in 
the  recent  Crania.  This  dorsal  foramen  was  described  and  fig- 
ured by  King13  in  1850,  Hall8  in  1860,  and  by  several  authors 
since,  and  has  commonly  been  termed  a  visceral  foramen. 

GEhlert21  suggests  that  it  was  probably  occupied  by  the 
terminal  portion  of  the  intestine.  The  persistence  of  the 
foramen  seems  to  indicate  an  anal  opening.  In  reference  to 
this  character  and  the  obsolescence  of  the  eyes  the  class 
must  be  viewed  as  retrogressive  since  Paleozoic  time.  Other 
features,  however,  are  manifestly  progressive;  namely,  the 
gradual  shortening,  through  time,  of  the  posterior  elements 
of  the  animal,  as  the  pedicle,  visceral  portions,  and  internal 
shell  structures,  and  the  expansion  of  the  anterior  parts,  as 
the  shell  and  brachia. 

A  further  advance  in  specialization  is  shown  in  the  limita- 
tion of  the  pedicle -opening  wholly  to  the  ventral  valve  in  the 
higher  rhynchonelloids,  athyroids,  spiriferoids,  and  terebratu- 
loids.  The  absence  of  punctse  in  all  the  early  radicles  and 
their  subsequent  development  in  the  derived  types  may  also 
have  a  similar  bearing. 


DEVELOPMENT  OF  THE  BRACHIOPODA  265 

The  features  and  importance  of  the  protegulum  have  pre- 
viously been  discussed.1  It  is  merely  noticed  here  as  the 
embryonic  shell  of  the  completed  phylembryonic  period,  for 
it  is  the  first  stage  which  can  be  observed  among  the  fossil 
species,  and  is  the  initial  point  for  the  discussions  of  the 
relations  and  affinities  of  recent  and  fossil  forms.  Of  the 
protegulum  and  later  stages,  there  is  abundant  material  avail- 
able in  nearly  every  family  of  brachiopods,  ranging  through 
their  entire  geological  history. 

Post-embryonic  Stages. 

In  discussing  the  post-embryonic  stages  of  growth  two 
aspects  of  development  must  be  clearly  differentiated ;  (a)  the 
ontogenetical,  and  (6)  the  phylogenetical.  The  ontogeny  of 
a  form  like  Schizocrania  may  be  conveniently  divided  into 
the  nepionic,  neanic,  and  ephebic  periods,  and  such  stages 
may  be  clearly  defined.  The  ephebic  stage  of  Schizocrania, 
however,  is  like  a  neanic  stage  of  Orbiculoidea.  In  other 
words,  Orbiculoidea,  in  its  development,  passes  through  a 
fSchizocrania-like  stage  before  reaching  maturity.*  These 
facts  must  be  viewed  from  a  phylogenetic  standpoint.  More- 
over, in  the  geological  history  of  a  group,  certain  ephebic 
characters  of  early  species  may  become  accelerated,  and  pass 
into  the  neanic  period  of  later  forms,  while  other  characters 
remain  ephebic.  Discinisca  offers  an  illustration  of  this. 
Its  neanic  characters  agree  with  Orbiculoidea  in  the  form  of 
the  valves  and  in  the  pedicle-notch,  but  the  circular  or  ellip- 
tical form  of  the  dorsal  valve  in  adult  and  neanic  Orbicu- 
loidea appears  so  early  in  Discinisca  that  it  marks  all  the 
nepionic  stages.  The  interpretation  of  these  facts  is,  of 
course,  very  evident,  and  will  be  subsequently  given  in 
detail.  Attention  is  here  called  to  the  statement,  that  while 
nepionic,  neanic,  and  ephebic  stages  represent  equal  intervals 

*  Attention  was  called  to  this  fact  in  a  publication  preliminary  to  vol.  viii  of 
the  Palaeontology  of  New  York,  pp.  131,  132,  issued  February,  1890.  Also,  the 
development  of  the  pedicle-opening  in  Orbiculoidea  was  fully  described. 


266  STUDIES  IN  EVOLUTION 

in  the  life  of  each  individual,  they  do  not  represent  condi- 
tions of  growth,  or  the  possession  of  characters  which  always 
agree,  stage  for  stage,  in  the  species  of  one  family  or  of 
different  families. 

Other  distinctions  to  be  made  whenever  possible  are  (a) 
whether  certain  characters  (natural  or  acquired)  belong  to  a 
species  by  inheritance,  or  (6)  are  mere  adaptations  to  special 
conditions  of  environment  arising  at  any  time  in  its  history. 
A  clear  understanding  of  the  first  will  lead  to  the  true  phylog- 
eny  of  a  species  or  genus,  but  to  reach  this  the  characters  of 
the  second  category  must  be  excluded.  Thus  in  the  series 
of  tSchizocrania,  Orbiculoidea,  and  Discinisca,  already  cited, 
there  is  an  apparent  genetic  connection  in  the  facts  as  stated. 
The  contrary  must  be  the  case  with  shells  like  Lingula 
complanata  Williams  and  L.  riciniformis  Hall,  which  initiate 
a  holoperipheral  *  mode  of  growth  in  the  ephebic  period,  for 
this  agreement  in  the  method  of  concrescence  with  adult 
Orbiculoidea  here  appears  in  the  mature  stages  of  this  species, 
and  being  absent  in  the  early  members  of  the  genus  cannot 
therefore  be  an  ancestral  character.  It  is  a  morphological 
equivalent,  which  may  or  may  not  be  continued  in  the  later 
species  of  the  series. 

Whenever  features  are  present  which  can  be  referred  to  an 
ancestral  origin,  their  elimination  can  take  place  only  by  the 
process  of  acceleration  of  development.  On  the  other  hand, 
there  may  be  secondary  characters  of  dynamical  or  homo- 
plastic  origin  which  appear  simultaneously  or  independently 
in  different  groups  belonging  to  diverse  genetic  lines,  as  the 
deltidial  plates  of  the  Rhynchonellidse,  Terebratulidae,  and 
Spiriferidse.  Further,  many  such  secondary  features  may 
occur  anywhere  in  the  geological  history  of  the  group,  as 
the  high  hinge-area  of  Orthisina,  Spirifer,  Syringothyris,  and 
Thecidium.  These  statements  are  in  full  accord  with  what 
Hyatt  has  determined  in  the  Cephalopoda,  and  the  applica- 
tion of  such  ideas  affords  a  fertile  field  of  research. 

*  8Aos  whole,  and  irfpupepfia  circumference. 


DEVELOPMENT  OF  THE  BRACHIOPODA  267 

Preliminary  to  a  study  of  the  stages  of  growth  observed 
in  the  different  orders,  a  simple  characteristic  example  of 
each  will  be  taken  to  show  the  limitations  of  the  post- 
embryonic  periods. 

Nepionic  Period.  —  In  brachiopods,  as  in  pelecypods,  this 
period  represents  the  growth  of  the  true  shell  immediately 
succeeding  the  embryonic  shell  or  protegulum,  and  before 
the  appearance  of  definite  specific  characters.  In  general, 
the  nepionic  shells  of  all  groups  are  marked  only  by  fine 
concentric  lines  of  growth,  and  are  therefore  nearly  smooth. 
Sometimes,  however,  a  few  radiating  striae  or  other  orna- 
ments may  appear  over  the  nepionic  portion,  but  this  is  not 
the  prevailing  rule.  Obolus  pulcher  Matthew  shows  a  can- 
cellated nepionic -stage  and  is  one  of  the  most  striking  excep- 
tional examples. 

Plate  XII,  figure  1,  represents  the  nepionic  stage  of  Glot- 
tidia  albida,  drawn  from  the  beak  of  a  well-preserved  adult. 
The  shell  at  this  period  had  a  short  straight  hinge  (originally 
the  hinge  of  the  protegulum),  with  lines  representing  ante- 
rior and  lateral  growth,  making  the  outline  broadly  ovate. 
It  is  divided  from  the  succeeding  growth  of  later  stages  by 
a  strong  varix.  The  form  is  suggestive  of  Obolella,  and  as 
this  is  the  early  form  of  growth  of  many  of  the  Lingulidse 
and  allied  families,  it  is  here  called  the  Obolella  stage.  It  is 
not  known  that  otherwise  the  characters  agree  with  those  of 
Obolella,  but  as  it  is  characteristic  as  well  as  descriptive  the 
name  is  used  to  designate  this  form  of  nepionic  growth  when- 
ever present. 

The  nepionic  stage  of  Orbiculoidea  minuta  (figure  4)  shows 
a  continuance  of  the  straight-hinged  condition  after  the  com- 
pletion of  the  embryonic  shell,  with  nearly  equal  incremental 
lines.  As  this  agrees  with  the  shell  of  Paterina  [=  Iphidea] 
it  is  called  the  Paterina  stage.  The  pedicle  emerged  freely 
between  the  cardinal  margins  of  the  valves.  It  will  be 
shown  that  both  this  and  the  Obolella  stage  are  represented 
in  the  nepionic  periods  of  many  genera  belonging  to  the 
Atremata.  They  may  succeed  each  other  in  a  single  species 


268  STUDIES  IN  EVOLUTION 

or   one   alone   may  be   present.     In   case   both   appear,   the 
Paterina  stage  is  always  the  first  one  to  be  developed. 

The  nepionic  stage  of  Leptcena  rhomboidalis  (figure  7, 
Plate  XII)  is  represented  by  a  shell  without  radii,  having 
a  comparatively  large  pedicle-opening  in  the  ventral  valve 
and  a  large  deltidium.  The  hinge  is  not  well  defined  and 
the  shell  is  discinoid  in  form.  This  term  is  not  used  to 
suggest  any  special  affinities  with  true  discinoid  genera,  as 
Orbiculoidea  or  Discinisca.  The  proper  name  for  this  stage 
is  not  yet  apparent  to  the  writer.  The  external  characters 
as  expressed  by  both  valves  are  manifestly  nearer  to  Kutor- 
gina  than  to  any  telotremate  genus.  Until  the  early  forms 
belonging  to  the  articulate  brachiopods,  especially  to  the 
orthoid  and  strophomenoid  groups,  have  been  thoroughly 
studied,  the  interpretation  of  the  nepionic  Leptcena  rhom- 
boidalis may  be  uncertain.  It  should  be  noted,  however, 
that  the  young  of  Chonetes,  Productus,  Stropheodonta,  Ortho- 
thetes,  Leptcena,  Plectambonites,  and  Strophomena,  all  have 
little  or  no  indication  of  a  straight  hinge -line,  and  that  the 
extension  of  this  member  takes  place  during  later  neanic 
and  ephebic  growth.  This  in  itself  is  significant,  but  is 
more  marked  when  taken  with  the  growth-stages  shown  by 
some  species  of  Strophomena  which  have  after  the  protegu- 
lum  a  Paterina-like  stage,  with  a  straight  hinge  in  the  dorsal 
valve,  succeeded  by  holoperipheral,  discinoid,  nepionic  growth, 
and  finally  a  renewal  of  a  straight-hinged  condition.  Thus  it 
has  an  early  straight-hinged  form,  which  is  lost  during  the 
next  stage  of  growth,  and  again  appears,  and  is  progressively 
elongated  during  neanic  and  ephebic  growth. 

The  nepionic  stages  of  Terebratulina  septentrionalis  (figure 
10,  Plate  XII)  represent  a  decreasing  extension  of  the  cardi- 
nal line  from  the  protegulum,  an  open  delthyrium,  the 
absence  of  radii,  and  the  introduction  of  the  shell  punctse. 
The  crura  at  this  stage,  as  shown  by  Morse,  are  short  and 
stout,  and  the  loop  is  undeveloped. 

Neanic   Period.  —  During  the  progress  of  this  period  all 


DEVELOPMENT  OF  THE  BRACHIOPODA  269 

the  features  which  reach  their  complete  growth  in  the 
adult  organism  are  introduced  and  progressively  developed. 
Usually  they  appear  in  succession,  and  gradually  assume 
mature  conditions.  Thus  in  many  species  with  radiate  plica- 
tions or  strise,  a  few  radii  appear  in  early  neanic  growth,  and 
are  added  to  until  the  full  number  is  present.  Species  with 
deltidial  plates  develop  them  in  this  period.  The  early 
stages  may  offer  many  points  for  comparison  with  the  adult, 
but  later  stages  usually  differ  little  except  in  size.  Figures 
2,  5,  8,  and  11,  Plate  XII,  represent  a  neanic  stage  in  each 
of  the  four  species  taken  as  examples.  Others  from  the 
same  species  could  be  given,  but  these  suffice  to  show  that 
one  or  more  characteristic  adult  features  have  made  their 
appearance. 

Ephebic  Period.  —  The  period  of  complete  normal  growth, 
or  the  maximum  of  individual  perfection.  This  corresponds 
to  the  adult,  or  mature  organism,  and  is  so  well  understood 
that  no  further  explanation  is  necessary.  For  the  sake  of 
completing  the  series,  the  .ephebic  shells  of  the  species 
given  are  represented  in  figures  3,  6,  9,  and  12,  Plate 
XII. 

Gerontic  Period.  —  The  variations  due  to  old  age  may  be 
numerous  and  complex.  As  shown  by  Clarke  and  the 
writer,3  the  valves  generally  become  thickened,  and,  as  a 
consequence,  the  margins  are  truncate  or  varicose,  the  ver- 
tical diameter  of  the  shell  is  increased,  the  beaks  involuted, 
and  the  margins  of  the  valves  often  lose  the  ornamentation 
characteristic  of  the  species.  The  deltidial  plates  or  del- 
tidium  may  be  resorbed  as  well  as  the  beaks  of  the  valves. 
Usually  the  ephebic  characters  disappear  in  inverse  order  to 
their  introduction.  Thus  in  a  normal  adult  brachiopod  hav- 
ing a  plicate  shell  and  deltidial  plates,  which  characters  were 
introduced  during  the  neanic  period,  the  expression  of  old 
age  will  be  found  in  the  absorption  of  the  deltidial  plates  and 
in  the  obsolescence  of  the  plications.  Large  specimens  of 
Terebratella  transversa  Sowerby  often  furnish  examples  of 
this  condition. 


270  STUDIES  IN  EVOLUTION 

The  gerontic  development  of  Bttobites*  consists  in  the 
obsolescence,  in  B.  various  Conrad,  of  the  bilobed  form  of  the 
shell,  thus  reverting  to  an  early  neanic  condition  equally 
characteristic  of  B.  bilobus  and  B.  Verneuiliamis. 

Another  aspect  of  growth  and  decline  is  manifest  when  the 
size  of  individuals  and  the  chronological  history  of  groups 
are  taken  into  consideration.  Each  genus  and  family  began 
with  small  representatives,  and  rapidly  developed  the  more 
radical  varieties  of  structure.  Then  came  the  culmination 
and  final  reduction  in  size,  with  abundance  of  gerontic  and 
pathologic  forms.  The  oldest  known  shell  with  calcareous 
spires,  Zygospira,  is  a  comparatively  minute  form.  Nearly 
all  the  types  of  the  sub-order  to  which  this  genus  belongs 
(Helicopegmata)  appear  in  the  Upper  Silurian.  Species  pre- 
senting the  maximum  size  belong  to  the  Devonian  and 
Carboniferous.  Before  the-  extinction  of  the  sub-order  in  the 
Trias,  the  individuals  are  small,  and  such  abnormal  genera  as 
Thecospira,  Koninckina^  and  AmpJiiclina  abound.  Productus 
begins  with  small  species  (Productella)  in  the  Lower  Devo- 
nian, and  in  the  Carboniferous  attains  the  largest  dimen- 
sions of  any  known  brachiopod  (P.  giganteus).  During 
the  Permian  the  species  have  dwindled  in  size,  and  the 
gerontic  Strophalosia  and  Aulosteges  are  the  chief  repre- 
sentatives. 

The  culmination  of  gerontic  growth  results  in  the  rever- 
sion of  the  animal  to  its  own  nepionic  period,  and  is  called 
the  paragerontic  stage.  As  this  is  an  extreme  condition,  it 
can  be  found  only  in  certain  genera  and  species  which  have 
been  developed  by  a  process  of  accelerated  gerontic  heredity. 
If  G-wynia  *  is  accepted  as  a  valid  genus,  it  belongs  to  a  pro- 
nounced paragerontic  type.  The  shell  has  a  small  internal 
plate  on  each  side  of  the  dorsal  umbo,  evidently  the  bases  of 
crural  plates.  King,14  the  author  of  the  genus,  states  that 

*  Some  authors  have  been  disposed  to  consider  this  form  as  the  young  of  a 
species  not  yet  determined.  It  has  also  been  referred  to  Macandrevia  cranium, 
Cistella  cistellula,  and  C.  neapolitana.  This  question  cannot  be  at  present  deter- 
mined, although  some  characters  of  the  shell  indicate  a  mature  organism. 


DEVELOPMENT  OF  THE  BRACHIOPODA  271 

the  labial  appendages  are  attached  directly  to  the  shell,  and 
not  to  a  loop,  as  in  other  genera  of  the  family.  Cistella  may 
be  taken  as  a  representative  of  paragerontic  development 
among  the  terebratuloids.  The  species  are  smooth  or  pau- 
ciplicate,  and  small;  deltidial  plates  obsolescent,  loop  more 
or  less  undeveloped.  In  0.  neapolitana  the  lamellae  of  the 
loop  are  nearly  obsolete  and  are  free  only  near  the  crura, 
while  the  anterior  portions  are  confluent  with  the  valve 
(Shipley).  A  slight  progression  of  these  reversions  would 
naturally  result  in  a  degenerate  form  like  Gwynia,  which  is, 
without  a  calcareous  loop;  with  no  surface  ornamentation; 
deltidial  plates  absent;  punctse  few  and  large,  all  of  which 
features  are  strictly  nepionic.  Besides  Cistella  and  Gwynia, 
other  loop-bearing  genera  present  paragerontic  features  of 
importance  in  a  natural  classification.  These  consist  mainly 
in  their  small  size;  the  absence  of  surface  ornaments;  the 
obsolescence  of  deltidial  plates,  and  the  loss  of  a  complete 
loop  supporting  the  arms.  In  the  Terebratulidaa  Kraussina 
and  Platydia  may  be  mentioned  as  belonging  to  gerontic 
types  with  a  paragerontic  tendency.  Likewise,  in  other 
groups,  Atretia  in  the  Rhynchonellidse,  and  Strophalosia  and 
Aulosteges  in  the  Productidas,  are  examples  of  paragerontic 
types. 

Cistella  and  G-wynia  among  the  genera  of  brachiopods, 
therefore,  bear  the  same  relation  to  the  terebratuloids  that 
Baculites  among  the  cephalopods  bears  to  the  ammonoids. 


Synopsis. 

Protembryo.  —  Ovum  and  segmented  stages  before  formation 
of  blastula  cavity. 

Mesembryo.  —  Blastosphere. 

Metembryo.  —  Gastrula. 

Neoembryo.  —  Trochosphere  and  cephalula,  with  posteri- 
orly directed  mantle  lobes,  and  bundles  of  setae  from  body 
segment. 

Typembryo.  —  Larva  with  mantle  lobes  folded  anteriorly  over 
head  segment. 


272  STUDIES  IN  EVOLUTION 

Phylembryo.  — -Bachiopod  covered  by  protegulum,  tentacles 
of  arms  developed,  bundles  of  setae  dehisced,  definition  of 
stomach  and  oesophagus,  direct  transformation  of  larval  muscles 
into  those  corresponding  to  muscles  of  adult  animal. 

Deltidium.  —  A  single  plate  developed  at  an  early  period  by 
the  body  and  pedicle  of  animal  posterior  to  dorsal  hinge,  and 
later  ankylosed  to  ventral  valve. 

Deltidial  Plates.  —  A  neanic  and  adult  feature  produced  by 
the  extensions  of  the  ventral  mantle  lobe  into  the  delthy- 
rium. 

Brachiopoda. — Retrogressive  in  loss  of  anal  opening  and  eyes, 
progressive  in  concentration  of  posterior  elements,  expansion 
of  anterior  elements,  and  limitation  of  pedicle-opening  to  one 
valve. 

Nepionic  Period.  —  Young  shells  before  the  appearance  of 
distinctive  specific  characters. 

Neanic  Period.  —  Progressive  development  of  the  specific 
features  which  reach  their  complete  growth  in  the  adult. 

Ephebic  Period.  —  Normal  adult  condition. 

Gerontic  Period.  —  Special  manifestations  of  old  age  in  ontog- 
eny and  in  phylogeny. 

Paragerontic  Types.  —  Extremes  of  geratology  represented  by 
Cistella,  Gwynia,  and  Atretia. 


References. 

1.  Beecher,  C.  E.,  1891.  —  Development  of  the  Brachiopoda.     Part  I. 

Introduction.     Amer.  Jour.  Sci.  (3),  vol.  xli,  April. 

2.   1891.— Development  of  Bilobites.     Amer.  Jour.  Sci.  (3),  vol. 

xlii,  July. 

3.   and  Clarke,  J.  M.,  1889.  —  The  Development  of  some  Silurian 

Brachiopoda.     Mem.  N.  Y.  State  Mus.,  vol.  i.  No.  1. 

4.  Brooks,  W.  K.,  1879.  —  The  Development  of  Lingula  and  the  Sys- 

tematic Position  of  the  Brachiopoda.    Johns  Hopkins  Univ.,  Chesa- 
peake Zool.  Lab.,  Sci.  Results,  Session  of  1878. 

5.  Davidson,   T.,   1851-1885. — A  Monograph  of  the  British  Fossil 

Brachiopoda.     Pal.  Soc.,  London. 

6.  Deslongchamps,  E.,  1862.  — Note  sur  le  developpement  du  deltidium 

chez  les  brachiopodes  articul£s.     Butt.    Soc.  Geol.  France  (2),  t. 
xix. 


DEVELOPMENT  OF  THE  BRACHIOPODA  273 

7.  Fewkes,  J.  W.,  1885.  —  On  the  Larval  Forms  of  Spirorbis  borealis, 

Daudin.     American  Naturalist,  March. 

8.  Hall,  James,  1860.  —  Palceontology  of  New  York,  vol.  iii. 

9.  Hyatt,  A.,  1888.  — Values  in  Classification  of  the  Stages  of  Growth 

and  Decline,  with  Propositions  for  a  New  Nomenclature.  Proc. 
Boston  Soc.  Nat.  Hist.,  vol.  xxiii,  March. 

10.   1889.  —  Genesis  of  the  Arietidae.    Mem.  Mus.  Comp.  Zool.,  vol. 

xvi,  No.  3. 

11.  Jackson,  R.  T.,  1890.  —  Phylogeny  of  the  Pelecypoda.     The  Avicu- 

lidse  and  their  Allies.  Mem.  Boston  Soc.  Nat.  Hist.,  vol.  iv, 
No.  viii. 

12.  Joubin,  L.,   1886.  —  Recherches  sur  1' Anatomic  des  Brachiopodes 

Inarticule's.     Archiv  Zool.  Experimentale  (2),  t.  iv. 

13.  King,  W.,  1850.  —  A  Monograph  of  the  Permian  Fossils  of  England. 

Pal.  Soc.,  London. 

14.  King,  W.,  1859.  —  On  Gwynia,  Dielasma,  and  Macandrevia,  three 

new  genera  of  Palliobranchiate  Mollusca,  one  of  which  has  been 
dredged  in  Belfast  Lough.  Proc.  Dublin  Univ.,  Zo'61.  Bot.  Assoc., 
vol.  i. 

15.  Kovalevski,  A.   O.,   1874.  —  Observations  on   the   Development  of 

Brachiopoda.  Proc.  Imp.  Soc.  Amateur  Naturalists,  etc.,  held  at 
the  University  of  Moscow,  llth  year,  vol.  xiv. 

16.  Lacaze-Duthiers,   H.,    1861.  —  I^istoire  naturelle   des    Brachiopodes 

vivants  de  la  Mediterranee.     Ann.  Sci.  Nat.  Zool.,  t.  xv. 

17.  Morse,  E.  S.,  1873.  —  On  the  Early  Stages  of  Terebratulina  septen- 

trionalis  (Couthouy).     Mem.  Boston  Soc.  Nat.  Hist.,  vol.  ii. 

18.   1873.  —  Embryology  of  Terebratulina.     Mem.  Boston  Soc.  Nat. 

Hist.,  vol.  ii. 

19.   1873.  —  On  the  Systematic  Position  of  the  Brachiopoda.  Proc. 

Boston  Soc.  Nat.  Hist.,  vol.  xv. 

20.  Miiller,  F.,  1860. —  Beschreibung  einer  Brachiopodenlarve.     Archiv 

Anat.  Physiol.,  Jahrg.  1860. 

21.  CEhlert,   D.    P.,    1887.  —  Brachiopodes.     Manuel  de    Conchyliologie, 

Paul  Fischer.     Appendice. 

22.  Shipley,  A.  E.,  1883.  —  On  the  Structure  and  Development  of  Argiope. 

Mittheil.  Zool.  Station  Neapel,  Bd.  IV. 

23.  Walcott,  C.  D.,  1888.  —  A  Fossil  Lingula  preserving  the  Cast  of  the 

Peduncle.     Proc.  U.  S.  Nat.  Mus. 


18 


274  STUDIES  IN  EVOLUTION 


PART  III.    MORPHOLOGY  OF  THE  BRACHIA* 

THE  diagnostic  value  of  the  brachidium,  or  calcareous  arm 
supports,  of  brachiopods  has  long  been  recognized,  and  forms 
one  of  the  chief  characters  for  generic  and  family  sub-division 
among  the  Terebratulacea  and  Spiriferacea.  This  character 
fails  in  all  other  brachiopods,  which  have  simply  fleshy  arms, 
unsupported  by  calcareous  skeletons.  There  is,  however, 
generally  the  most  obvious  analogy  and  intimate  relationship 
between  the  arms  themselves  and  the  brachidium,  so  that 
whenever  either  structure  can  be  ascertained  it  furnishes 
important  data  aiding  in  the  determination  of  the  systematic 
position  of  any  genus  within  a  family  or  order. 

The  growth  of  the  arms,  or  lophophore,  in  recent  genera 
may  be  divided  into  distinct  stages,  which  often  have  a  direct 
correlation  with  other  important  features  of  the  shell.  In 
many  cases  it  is  also  possible  to  infer  the  form  and  arrange- 
ment of  the  brachia  in  fossil  genera  from  markings  on  the 
interior  of  the  valves  and  from  the  calcareous  arm  supports, 
and  thus  to  obtain  the  chronogenetic  as  well  as  the  morpho- 
genetic  history  of  these  organs. 

The  most  detailed  accounts  of  arm  development  are  given 
by  Brooks  5  f  for  Grlottidia,  by  Morse  n  for  Terebratulina,  and 
by  Kovalevski10  for  Cistella  and  Thecidea.  These  results, 
combined  with  original  observations  by  the  writer  *• 2  and 
occasional  descriptions  of  arm  structure  by  Davidson  7  and 
other  authors,  are  sufficient  to  include  and  to  interpret  prop- 
erly all  the  leading  varieties  of  structure. 

As  shown  by  Brooks  5  the  tentacles,  or  cirri,  in  Grlottidia 
originate  on  the  dorsal  side  of  the  oral  disk.  They  grow  in 
pairs,  one  on  each  side  of  a  central  lobe.  New  tentacles 
are  added  between  the  first  pair  formed  and  the  median  lobe. 

*  Bulletin  87,  U.  S.  Geol.  Surv.,  Chapter  IV,  105-112,  1897. 

t  The  references  to  the  literature  will  be  found  at  the  end  of  this  chapter. 


DEVELOPMENT  OF  THE  BRACHIOPODA  275 

Thus  the  cirri  farthest  removed  from  the  median  lobe  are  the 
oldest.  Tentacles  are  added  rapidly  until  the  first  arc  is  ex- 
tended to  a  semi-circle,  and  then  progressively  the  whole  disk 
becomes  surrounded  by  a  circle  of  these  organs.  The  further 
introduction  of  cirri  can  take  place  only  by  the  enlargement 
of  the  oral  disk  or  through  the  deformation  of  the  circle  by 
lobes,  loops,  or  extensions.  In  &lottidia,  Lingula,  Discinisca, 
Crania,  and  Rhynchonella  the  two  points  of  tentacular  in- 
crease, originally  together  and  on  opposite  sides  of  a  median 
lobe,  or  tentacle,  gradually  separate,  and  the  further  multi- 
plication of  tentacles  results  in  strap-shaped  extensions  on 
each  side,  which  finally  assume  a  coiled  form,  due  to  the 
limited  space  in  which  they  grow.  Therefore  the  arms 
in  adult  individuals  of  these  genera  have  a  single  cirrated 
edge,  extending  from  their  free  extremities  to  the  sides  of 
the  oral  disk,  and,  continuing  posteriorly,  unite  on  the  ven- 
tral side  of  the  disk  behind  the  mouth.  Each  cirrated  edge 
in  the  adult  lophophore  apparently  has  two  approximate  rows 
of  alternating  cirri  (Hancock9,),  but  as  they  were  originally 
a  single  row  in  early  stages,  this  appearance  is  evidently 
the  result  of  a  crowding  of  the  cirri  or  a  crumpling  of  the 
edge. 

Kovalevski 10  has  shown  that  in  Cistella  the  tentacles  also 
originate  in  pairs  on  each  side  of  the  dorso-median  line,  with- 
out a  central  tentacle  or  lobe.  The  same  mode  of  increase 
has  been  shown  by  the  writer2  to  be  present  in  Magellania 
and  Terebratalia.  In  young  stages  of  Cistella,  Terebratulina, 
Magellania,  and  other  terebratuloid  genera,  as  well  as  in  The- 
cidea,  after  the  circlet  of  tentacles  is  complete  the  two  points  at 
which  new  ones  are  added  do  not  separate,  but  remain  close 
together  throughout  the  life  of  the  animal.  In  this  case  the 
cirrated  margin  is  lengthened  by  means  of  lobation  and  loop- 
ing, and  often  by  the  final  growth  of  a  single,  median,  coiled 
arm,  cirrated  on  both  margins.  G-wynia  illustrates  the  com- 
pleted circle  of  tentacles  about  the  mouth.  Adult  Cistella 
shows  an  advance  in  having  the  anterior  margin  of  the  lopho- 
phore introverted,  making  it  bilobed.  Megathyris  is  slightly 


276  STUDIES  IN  EVOLUTION 

more  complicated  by  two  additional  lobes.  This  simple 
method  of  increase  is  further  elaborated  in  the  Thecidiidse. 
In  the  higher  genera,  especially  among  the  Terebratulidse, 
the  maximum  is  reached  by  means  of  a  median,  unpaired, 
coiled  arm,  as  in  Magellania  and  Terebratulina. 

The  development  of  the  different  types  and  varieties  of 
arm  structure  is  presented  in  the  accompanying  figures  (121- 
125),  which  are  necessarily  somewhat  diagrammatic  in  order 
to  show  the  features  clearly,  but  the  essential  structure  can 
be  readily  verified  from  consultation  of  the  works  cited  or 
from  a  study  of  actual  specimens.  In  the  case  of  fossil  forms, 
such  as  Dielasma,  the  Atrypidse,  and  Athyridse,  the  brach- 
ial  supports  have  sufficient  analogy  with  the  arm  structures 
of  Terebratulina  and  Rhynchonella  to  warrant  their  interpreta- 
tion as  given.  Also,  the  spiral  impressions  on  the  valves  of 
Davidsonia,  and  those  occasionally  present  in  Leptcena  and 
Producing  clearly  point  to  the  possession  of  coiled  arms  by 
these  genera. 


Classification  of  Brachial  Structures. 

From  what  has  already  been  shown  it  is  seen  that  the 
various  types  of  lophophore  admit  of  a  simple  classification 
into  stages  and  groups.  It  is  proposed  to  give  to  these 
distinctive  names,  which  may  be  used  with  facility  in  making 
comparisons  and  correlations.  They  may  be  found  useful, 
also,  in  designating  the  kind  of  brachial  complexity  attained 
in  any  genus  the  arm  structure  of  which  can  be  determined, 
thus  helping  to  fix  its  place  in  a  genetic  scale.  It  should  be 
emphasized,  however,  that  the  form  and  complexity  of  the 
cirrated  margin  of  the  lophophore  can  have  a  taxonomic  value 
only  within  comparatively  narrow  limits.  This  at  once  be- 
comes evident  when  the  arms  of  Lingula,  Discinisca,  Crania, 
Rhynchonella,  and  all  the  Spiriferacea  are  considered.  Each 
has  spiral  arms,  which  were  probably  developed  through 
similar  changes  of  form,  and  yet  each  is  genetically  distinct, 


DEVELOPMENT  OF  THE  BRACHIOPODA  277 

as  shown  by  all  the  other  leading  characters.  But  when  this 
classification  of  arm  structures  is  applied  within  a  family  or 
genus,  or  even  when  made  the  basis  of  comparison  among  some 
closely  related  families,  it  is  sometimes  possible  to  reach  very 
satisfactory  conclusions  relating  to  the  systematic  position  of 
various  forms. 

Leiolophus  Stage. 

It  is  hardly  necessary  to  direct  attention  to  the  embryonic 
brachial  structure  before  the  growth  of  any  of  the  tentacles, 
or  cirri,  on  the  edge  of  the  lophophore,  while  the  animal  is  in 
the  typembryonic  stage.  For  the  sake  of  designating  all  the 
stages,  this  may  be  called  the  leiolophus  stage,  though  it  has 
110  special  significance  beyond  indicating  the  beginning  of  the 
lophophore. 

TaxolopJius  Stage. 

The  first  stage  in  which  a  true  brachial  structure  is  mani- 
fest is  an  early  larval  form,  often  the  protegulum  stage,  when 
the  tentacular  portion  of  the  lophophore  is  a  simple  arc  or 
crescent.  This  may  be  called  the  taxolophus.  The  tenta- 
cles are  few  in  number,  and  increase  takes  place  on  each 
side  of  the  median  line,  dorsally,  in  front  of  the  mouth. 
In  figures  121,  a,  e,  122,  a,  /,  124,  a,  this  character  is  clearly 
shown.  The  tentacles  at  the  ends  of  the  arc  are  the  oldest, 
and  new  ones  are  being  formed  in  the  middle  portion. 
In  Thecidea,  Cistella,  and  Magellania  the  tentacles  of  the  taxo- 
lophus are  centripetal,  due  to  the  edge  of  the  lophophore  being 
near  the  margin  of  the  shell;  while  in  Terelratulina,  Disci- 
nisca,  and  Lingula  they  are  centrifugal,  due  to  the  smaller 
and  central  lophophore. 

So  far  as  known,  there  is  no  adult  living  form  which  has 
the  taxolophian  brachial  structure.  It  may  have  been  pres- 
ent in  adult  Iphidea  of  the  Cambrian. 


278  STUDIES  IN  EVOLUTION 

Trocholophus  Stage. 

By  the  continual  addition  of  new  cirri  and  the  pushing 
back  of  the  old  ones,  the  fringed  margin  of  the  lophophore 
passes  from  a  crescentic  to  a  circular  form,  thus  making  a 
complete  ring  about  the  mouth.  This  may  be  termed  the 
trocholophus  stage.  It  appears  in  the  late  larval  and  early 
adolescent  stages  of  Thecidea  (figure  121,  6),  Cistella  (figure 
121,/),  Magellania  and  Terabratalia  (figure  122,  5),  Terebratu- 
lina  (figure  122,  #),  Grlottidia  (figure  124,  5),  and  Discinisca, 
and,  like  the  former  stages,  is  undoubtedly  common  to  all 
brachiopods,  except,  perhaps,  Iphidea. 

G-wynia  is  an  adult  living  representative  of  this  stage,  and 
never  develops  any  higher  type  of  brachial  structure.  Dy&- 
colia  also  belongs  here,  since  it  has  a  discoid  lophophore 
surrounded  by  a  marginal  fringe  of  tentacles  (Fischer  and 
(Ehlert8).  It  is  possibly  a  little  more  advanced  than  G-wynia, 
as  it  has  a  slight  median  anterior  notch,  suggesting  the  begin- 
ning of  the  bilobed  structure  of  the  next  higher  type. 

The  absence  of  septum,  hinge-plate,  and  dental  plates  are 
other  primitive  characters  belonging  to  Dyscolia. 


Schizolophus  Stage. 

After  the  completion  of  the  trocholophus  stage  in  all  brachi- 
opods, except  such  simple  forms  as  Gwynia  and  Dyscolia,  no 
further  increase  in  the  cirrated  edge  of  the  lophophore  can 
occur  without  some  deformation  of  the  circle.  This  is  first 
accomplished  by  an  introversion  of  the  anterior  median  edge, 
thus  dividing  the  lophophore  into  two  lobes,  and  suggesting 
the  name  schizolophus  for  this  type.  (See  figures  121,  c,  g, 
122,  c,  h,  124,  c.) 

Several  brachiopods  retain  the  schizolophian  brachia  as 
an  adult  character.  Of  these,  Cistella  is  perhaps  the  best 
example,  as  it  agrees  exactly  with  an  early  stage  of  arm 
structure  among  the  Terebratellidse,  which  has  been  called 
the  cistelliform  stage  (figure  122,  c?).  Terelratulina  (figure 


DEVELOPMENT  OF  THE  BRACHIOPODA  279 

122,  A),  G-lottidia  (figure  124,  c\  and  other  higher  forms  also 
have  corresponding  schizolophian  stages,  but  are  without  the 
median  septum.  Lacazella  mediterranea  presents  a  similar 
larval  structure,  and  in  L.  Barretti  it  is  retained  to  maturity. 
The  fossil  genera  Davidsonella  and  Thecidella  of  the  The- 
cidiidsB,  and  Zellania  of  the  Terebratellida3,  never  developed 
beyond  the  schizolophus -stage,  and  they  must  therefore  be 
considered  as  quite  primitive  genera  in  their  respective 
families. 

121 

v^      $ 

Taxolophus. 


Trocholophus. 


Schizolophus. 


Ptycholophus. 


FIGURE  121. —  Stages  of  growth  of  the  lophophore  in  Thecidea,  Cistella,  and 
Megaihyris.  a,  b,  c,  d,  stages  in  the  growth  of  the  lophophore  in  Thecidea 
(Lacazella)  mediterranea.  Enlarged,  (a-c,  after  Kovalevski;  d,  after  Lacaze- 
Duthiers.)  e,  ft  early  stages  of  lophophore  of  Cistella  neapolitana.  Enlarged. 
(After  Kovalevski.)  g,  adult  lophophore  of  Cistella  (C.  cistellula).  Enlarged. 
(After  Davidson.)  h,  labial  appendages  of  Megaihyris  decollata.  Enlarged. 
(After  Davidson.) 

From  this  point  the  further  development  and  complication 
of  arm  structure  proceeds  in  three  distinct  diverging  lines, 
producing  the  three  characteristic  types  of  brachia  of  all  the 
higher  brachiopods,  as  exemplified  in  Thecidea^  Terebratulina, 
and  Rhynchonella. 


280 


STUDIES  IN  EVOLUTION 


Ptycholophus  Stage. 

The  simplest  of  the  types  of  brachia  just  cited  is  developed 
out  of  the  schizolophus  by  the  additional  lobation,  or  loop- 
ing, of  the  primary  lobes,  making  a  structure  which  may  be 


Taxolophus. 


Trocholophus. 


Schizolophus. 


Zugolophus. 


Plectolophus. 


FIGURE  122.  —  Stages  of  growth  of  the  lophophore  in  the  Terebratellidje  and 
Terebratulidae.  a,  6,  c,  d,  e,  five  stages  in  the  development  of  the  lophophore  in 
the  Terebratellidae.  a-d,  Terebratalia  obsoleta.  Enlarged.  (After  Beecher.2) 
e,  Magellania  kerguelenensis.  Natural  size.  (After  Davidson.7)  /,  g,  h,  i,j,  develop- 
ment of  lophophore  in  the  Terebratulidae.  /-»,  early  stages  in  Terebratulina 
septentrionalis.  Enlarged.  (After  Morse.11)  /,  adult  Terebratulina  cancellata. 
(After  Davidson.7) 

called  the  ptycholophus.  Megathyris  and  Lacazella  mediter- 
ranea  both  have  four  lobes  (figure  2,  d,  £);  Thecidea  radiata 
has  six;  T.  vermicularis  and  Eudesella  may  ale,  eight; 
E.  digitata,  ten;  Pterophloios  and  Oldhamina,  about  twenty. 
Lobation  in  some  (Ttiecided)  is  produced  by  the  forking  or 


DEVELOPMENT  OF  THE  BRACHIOPODA  281 

branching  of  the  median  septum;  in  others  (Pterophloios) 
the  septum  remains  simple,  while  the  lateral  borders  of 
the  lophophore  are  lobed. 

Zugolophus  and  Plectolophus  Stages. 

All  the  higher  Terebratulacea  reach  the  final  growth  of  the 
lophophore  through  an  intermediate  stage  which  from  its 
form  may  be  called  the  zugolophus  (figure  122,  d,  i).  Eucala- 
this  and  Platidia  (?  Tropidoleptus)  are  apparently  adult  rep- 
resentatives of  this  stage,  while  Kraussina  and  probably 
Bouchardia  are  slightly  more  advanced  by  the  growth  of  a 
short  median,  coiled  arm,  and  lead  to  the  next  higher,  or 
plectolophus,  stage,  in  which  there  is  a  well-developed  spiral 
arm  with  a  fringe  of  cirri  on  each  edge  (figure  122,  e,  /). 

A  long  loop  pointed  in  front,  like  Rensselceria  and  Centra- 
nella,  could  not  have  supported  a  median  arm,  as  the  pallial 
cavity  is  thus  fully  occupied,  and  the  development  of  the 
brachidium  in  the  Terebratellidse  shows  that  the  central  space 
between  the  branches  of  the  loop  is  to  accommodate  such  an 
organ.  The  same  is  doubtless  true  of  Dielasma,  which  first 
has  a  Centronetta-\ike  loop,  and  through  the  subsequent 
resorption  of  the  anterior  portion  the  ascending  branches 
are  formed  and  space  allowed  for  the  median  arm  (figure 
123,  a-d).  In  a  spire-bearing  genus  like  Zygospira  this  is 
more  obvious,  for  here  the  transverse  process  or  jugum  is 
clearly  the  result  of  the  growth  and  resorption  of  the  cen- 
tronelliform  loop  to  admit  the  spiralia. 

123 


FIGURE   123.  —  Metamorphoses  of   the  brachidium  in  Dielasma  turgidum. 
Enlarged.     (After  Beecher  and  Schuchert.) 

The  calcareous   loop  in   Terebratulina  and  Liothyrina   is 
only  a  posterior  basal  support,  and  does  not  repeat  the  out- 


282 


STUDIES  IN  EVOLUTION 


line  of  the  cirrated  margin  of  the  lophophore,  exclusive  of 
the  arm.  Therefore  it  is  impossible  in  these  and  closely 
allied  genera  to  infer  the  stage  of  development  of  the  lopho- 
phore from  the  loop  alone.  Dyscolia  is  an  excellent  example, 
124_  since  the  loop  is  the  same  as  in 

Terebratulina  ;  but  the  lophophores 
are  quite  distinct  in  each,  the 
former  being  of  the  trocholophus 
type  and  the  latter  belonging  to  the 
Trocholophus.  plectolophus. 


Taxolophus. 


Spirolophus  Stage. 

The  last   type  to  be  noticed  is 
the   one   in   which    there   are   two 

Schizolophus.  -T    -I  i 

separate  coiled  arms,  each  with  a 
row  of  cirri  on  one  edge  only  (fig- 
ure 124,  d,  e).  It  embraces  the 
greater  part  of  the  families  of  brach- 
Spirolophus.  iopods  in  the  orders  Telotremata 
and  Protremata,  and  includes  all 
the  living  species  in  the  orders 
Atremata  and  Neotremata. 

In  the  early  stages  of  develop- 
ment of  the  spiral  lophophore  there 
is   an    agreement    with    the    early 
FIGURE  124.  —  Early  stages    stages  of  the  families  alreadv  no- 

of  lophophore  of  Glottidia,  and     ,•       i  -,    .,  ,      ,  , 

adult  brachia  in  Lingula  and  tlCed'  and  the  taxolophus,  trochol- 
Hemithyris.  a,  b,  c,  early  stages  Ophus,  and  Schizolophus  Stages  may 

Brooks.)    d,  adult  brachia  in    The  separation  and  growth  of  the 

Lingula.  (After  Woodward.)  spiral  arms  seem  to  be  due  to  the 
e,  adult  brachia  in  Hemithyris  .  -.  . 

psittacea.    (After  Hancock.)        widening  or  expansion  of  the  me- 
dian lobe  or  tentacle,  on  each  side 

of  which  is  the  formative  tissue  for  new  cirri.  This  is  very 
apparent  in  the  young  Ducinisca  described  by  Muller,12  and 
the  G-lottidia  described  by  Brooks.6 

The  brachidium  in  Zygospira  passes  through  a  series  of 


DEVELOPMENT  OF  THE  BRACHIOPODA  283 

changes  which  have  been  described  in  detail  elsewhere.* 
These  metamorphoses  are  of  great  assistance  in  understand- 
ing the  development  and  comparative  morphology  of  this 
feature  in  other  groups  of  the  Spiriferacea.  The  earliest 
stage  observed  (figure  125,  a)  has  the  form  of  a  simple  tere- 
bratuloid  loop,  which,  from  its  resemblance  to  Centronella, 
was  called  the  centronelliform  stage.  Since  approximately 
this  form  of  brachidium  is  also  characteristic  of  the  young 
of  recent  terebratuloids,  it  may  be  taken  in  Zygospira  as 
indicative  of  the  trocholophus  stage  of  brachial  development. 
With  this  as  a  starting-point  for  comparison,  the  further 
correlation  of  the  succeeding  stages  is  very  simple. 

The  first  resorption  of  the  end  of  the  loop  in  Zygospira 
produced  a  schizolophus  condition,  and  further  resorption 
carried  the  brachidium  to  a  stage  closely  resembling  Dielasma 
(figure  125,  6).  The  dielasmatiform  stage  has  already  been 
explained  as  due  to  the  requirements  of  space  for  the  growth 
of  the  coiled  brachia.  Next,  the  initial  calcification  of 
the  spiral  arms  resulted  in  i^ie  extension  of  the  descending 
branches  beyond  the  jugum  (figure  125,  <?),  and,  lastly,  com- 
plete calcification  manifests  the  spirolophus  structure  and 
produced  the  characteristic  brachidium  of  the  Spiriferacea. 

The  Atrypidse  and  the  Athyridse  seem  to  stand  to  each 
other  in  the  same  relation  as  the  Terebratellidse  and  Tere- 
bratulidse.  In  the  first  the  descending  branches  are  widely 
separated  and  follow  the  edges  of  the  valves;  in  the  second 
the  descending  branches  are  close  together.  This  difference 
in  the  Spiriferacea  produces  the  converging  cones  of  the 
Atrypidse  (figure  125,  cT)  and  the  diverging  cones  of  the 
Athyridse,  Spiriferidee  (figure  125,  <?),  etc. 

It  seems  doubtful  whether  the  fleshy  portions  of  the  brachia 
in  the  Meristellidse  and  Athyridse  possessed  additional  char- 
acters expressing  the  complexity  and  elaboration  reached  by 
the  jugal  processes,  even  when  the  lamellae  were  duplicated, 
as  in  Koninckina  and  Kayseria. 

From  the  foregoing  descriptions  and  illustrations  it  appears 
that  the  mode  of  growth  of  the  cirrated  lophophore,  or 


284  STUDIES  IN  EVOLUTION 

brachia,  is  alike  in  the  larval  stages  of  all  brachiopods. 
They  first  develop  tentacles  in  pairs  on  each  side  of  the 
median  line  in  front  of  the  mouth  (taxolophus  stage).  New 
tentacles  are  continually  added  at  the  same  points,  until,  by 
pushing  back  the  older  ones,  they  form  a  complete  circle 
about  the  mouth  (trocholophus  stage),  later  becoming  intro- 
verted in  front  (schizolophus  stage).  From  this  common  and 
simple  structure  all  the  higher  types  of  brachial  complication 
are  developed  through  one  of  two  methods :  (1)  The  growing 

125 


FIGURE  125. — Metamorphoses  of  brachidium  of  Zygospira  and  adult  bra- 
chidium  of  Rhynchospira.  a,  b,  c,  d,  metamorphoses  of  brachidium  of  Zygospira 
recitrvirostris.  Enlarged.  (After  Beecher  and  Schuchert.)  e,  brachidium  of 
Homceospira  evax.  (After  Beecher  and  Clarke.) 

points  of  the  lophophore,  or  points  at  which  new  tentacles 
are  formed,  remain  in  juxtaposition;  or  (2)  they  separate. 
Complexity  in  the  first  is  produced  (a)  by  lobation,  as  in 
Megathyris,  Eudesella,  Pterophloios  Thecidea,  etc.  (ptycholo- 
phus  type),  and  (b)  by  looping  (zugolophus)  and  the  growth 
of  a  median,  unpaired  coiled  arm  (plectolophus),  as  in  Magel- 
lania,  Terebratulina,  etc. ;  in  the  second  (c)  by  the  growth  of 
two,  separate,  coiled  extensions  or  arms,  one  on  each  side 
of  the  median  line  (spirolophus),  as  in  Lingula,  Crania,  Dis- 
cinisca,  Rhynchonella,  Leptcena,  Davidsonia,  Spirifer,  Athyris, 
Atrypa,  etc. 


DEVELOPMENT  OF  THE  BRACHIOPODA  285 


References. 

1.  Beecher,  C.  E.,  1893.  —  Revision  of  the  families  of  loop-bearing 

Brachiopoda.     Trans.  Conn.  Acad.  Sci.,  vol.  ix. 

2.    1893.  —  The  development  of  Terebratalia  obsoleta  Dall.     Trans. 

Conn.  Acad.  Sci.,  vol.  ix. 

3.    and  J.  M.  Clarke,  1889. — The  development  of  some  Silurian 

Brachiopoda.     Mem.  N.  Y.  State  Mus.,  vol.  i,  No.  1. 

4.    and  Charles  Schuchert,  1893.  —  Development  of  the  brachial 

supports  in  Dielasma  and  Zygospira.    Proc.  Biol.  Soc.  Washington, 
vol.  viii. 

5.  Brooks,   W.   K.,   1879.  —  The   development  of    Lingula    and    the 

systematic  position  of  the  Brachiopoda.  Johns  Hopkins  Univ., 
Chesapeake  Zool.  Lab. 

6.  Davidson,   T.,    1851-1885.  —  A   monograph   of  the    British    fossil 

Brachiopoda.     Pal.  Soc.,  London. 

7.    1886-1888.  —  A  monograph   of  recent   Brachiopoda.     Trans. 

Linn.  Soc.,  London,  vol.  iv. 

8.  Fischer,    P.,  and  D.-P.  (Ehlert,    1892.  —  Re'saltats   des  campagnes 

scientifiques  accomplies  sur  son  yacht  par  Albert  ler,  Prince 
Souverain  de  Monaco.  Fascicule  III.  Brachiopodes  de  1'Atlan- 
tique  Nord. 

9.  Hancock,   A.,   1858.  —  On  the  organization   of    the   Brachiopoda. 

Phil.  Trans.,  vol.  cxlviii. 

10.  Kovalevski,    A.   O.,    1874. — Observations   on   the   development   of 

Brachiopoda.  Proc.  Imp.  Soc.  Amateur  Naturalists,  etc.,  held  at 
the  University  of  Moscow,  llth  year,  vol.  xiv. 

11.  Morse,  E.  S.,  1873.  —  On  the  early  stages  of  Terebratulina  septentrio- 

nalis  (Couthouy).     Mem.  Boston  Soc.  Nat.  Hist.,  vol.  ii. 

12.  Miiller,  F.,  1860.  —  Beschreibung  einer  Brachiopodenlarve.     Archiv 

nat.  PhysioL,  Jahrg.  1860. 


2.  SOME  CORRELATIONS  OF  ONTOGENY  AND 
PHYLOGENY  IN  THE  BRACHIOPODA* 

(PLATE  XIII) 

THE  parallelism  between  the  ontogeny  and  phylogeny  in 
the  Brachiopoda  has  been  worked  out  in  numerous  instances,  f 
To  illustrate  these,  some  more  or  less  familiar  genera  may  be 
taken  as  characteristic  examples. 

Lingula  has  been  shown  by  Hall  and  Clarke  (Pal.  N.  K, 
vol.  viii,  1892)  to  have  had  its  inception  in  the  Ordovician. 
In  the  ontogeny  of  both  recent  and  fossil  forms,  the  first 
shelled  stage  has  a  straight  hinge-line,  nearly  equal  in  length 
to  the  width  of  the  shell.  This  stage  may  be  correlated 
with  the  more  ancient  genus  Paterina  [—  IpMdeci],  from  the 
lowest  Cambrian.  Subsequent  growth  produces  a  form  re- 
sembling Obolella,  a  Cambrian  and  Lower  Silurian  genus. 
Then  the  linguloid  type  of  structure  appears  at  an  adolescent 
period,  and  is  completed  at  maturity.  Thus  Lingula  has 
ontogenetic  stages  corresponding  to  (1)  Paterina  [=  Iphidea~], 
(2)  Obolella,  and  (3)  Lingula,  of  which  the  first  two  occur  as 

-  American  Naturalist,  XXVII,  599-604,  pi.  xv,  1893. 

t  C.  E.  Beecher.  Development  of  the  Brachiopoda.  Part  I.  Introduction. 
Amer.  Jour.  Set.,  XLI,  April,  1891. 

Development  of  the  Brachiopoda.     Part  II.     Classification  of  the  Stages 

of  Growth  and  Decline.    Amer.  Jour.  Sci.,  XLIV,  August,  1892. 

Development  of  Bilobites.     Amer.  Jour.  Sci.,   XLII,  July,  1891. 

Revision  of  the  Families  of  Loop-bearing  Brachiopoda.     Trans.  Conn. 

Acad.  Sci.,  IX,  May,  1893. 

Deslongchamps,  E.  Etudes  critiques  sur  des  Brachiopodes  nouveaux  ou  peu 
connus,  1884. 

Fischer  and  CEhlert.  Brachiopodes :  Mission  Scientifique  du  Cap  Horn, 
1882-1883.  Bull.  Soc.  Hist.  Nat.  d'Autun,  V,  1892. 


^ 

ONTOGENY  AND  PHYLOGENY  IN  BRACHIOPODA   287 

adult  forms  in  geological  formations  older  than  any  known 
Lingula. 

Paterina  [=  Iphidea]  represents  the  prototype  of  the  brachi-  I 
opods.  It  shows  no  separate  stages  of  growth  in  the  shell,  j 
is  found  in  the  oldest  fossil  if  erous  rocks,  and  corresponds  to  ' 
the  embryonic  shelled  condition  (protegulum)  of  the  class. 

The  genus  Orbiculoidea  of  the  Discinidse  first  appears  in 
the  Ordovician  and  continues  through  the  Mesozoic.  The 
early  stages  in  the  ontogeny  of  an  individual  are,  as  in 
Lingula,  first  a  Paterina  stage,  followed  by  an  Obolella  stage. 
Then,  from  the  mechanical  conditions  of  growth,  a  Schizo- 
crania-likQ  stage  follows,  and  complete  growth  results  in 
Orbiculoidea. 

The  elongate  form  of  the  shell  in  Lingula  as  well  as  in 
many  other  genera  is  determined  by  the  length  of  the  pedicle 
and  freedom  of  motion.  The  discinoid,  or  discoid,  form  of 
Orbiculoidea  and  Discinisca  among  the  brachiopods,  and 
Anomia  among  pelecypods,  is  determined  by  the  horizontal 
position  of  the  valves,  which  are  attached  to  an  object  of  sup- 
port by  a  more  or  less  flexible,"  very  short  organ,  —  a  pedicle 
or  byssus,  without  calcareous  cementation.  This  mode  of 
growth  is  characteristic  of  all  the  discinoid  genera,  but,  as 
already  shown,  the  early  stages  of  Paleozoic  Orbiculoidea  have  \ 
straight  hinge-lines  and  marginal  beaks,  and  in  the  adult 
stages  of  the  shell  the  beaks  are  usually  sub-central  and  the 
growth  holoperipheral.  This  adult  discinoid  form,  which 
originated  and  was  acquired  through  the  conditions  of  fixa- 
tion of  the  animals,  has  been  accelerated  in  the  recent  Dis- 
cinisca so  that  it  appears  in  a  free-swimming  larval  stage. 
Thus,  a  character  acquired  in  adolescent  and  adult  stages 
of  Paleozoic  species,  through  the  mechanical  conditions  of 
growth,  appears  by  acceleration  in  larval  stages  of  later  forms 
before  the  assumption  of  the  condition  of  fixation  which  first 
produced  this  character. 

The  two  chief  sub-families  of  the  Terebratellidse  undergo 
complicated  series  of  metamorphoses  in  their  brachial  struc- 


^ 
0 


288  STUDIES  IN  EVOLUTION 

ture.  Generic  characters  in  this  family  are  usually  based 
upon  the  form  and  disposition  of  the  brachia  and  their  sup- 
ports. The  highest  genera  in  one  sub-family,  which  is  austral 
in  distribution,  pass  through  stages  correlated  with  the  adult 
structure  in  the  genera  G-wynia,  Cistella,  Bouchardia,  Meger- 
lina,  Magas,  Magasella,  and  Terebratella,  and  reach  their 
final  development  in  Magellania  and  Neothyris.  The  higher 
genera  in  another  sub -family,  boreal  in  distribution,  pass 
through  metamorphoses  correlated  with  the  adult  structures 
of  G-wynia,  Cistella,  Platidia,  Ismenia,  Muhlfeldtia,  Terebra- 
talia,  and  Dallina.  The  first  two  stages  in  both  sub-families 
are  related  in  the  same  manner  to  G-wynia  and  Cistella.  The 
subsequent  stages  are  different  except  the  last  two,  so  that 
the  Magellania  structure  is  similar  in  all  respects  to  the  Dal- 
lina structure,  and  Terebratella  is  like  Terebratalia.  There- 
fore Magellania  and  Terebratella  are  respectively  the  exact 
morphological  equivalents  to,  or  are  in  exact  parallelism 
with,  Dallina  and  Terebratalia. 

The  stages  of  growth  of  the  genera  belonging  to  the  two 
sub-families  Dallininse  and  Magellaniinee  are  further  corre- 
lated in  the  tables  on  page  303. 

The  simplest  genus,  Grivynia,  as  far  as  known,  passes 
through  no  brachial  metamorphoses,  and  has  the  same  struc- 
ture throughout  the  adolescent  period,  up  to  and  including 
the  mature  condition.  In  the  ontogeny  of  Cistella  the 
gwyniform  stage,  through  acceleration,  has  become  a  larval 
condition.  In  Platidia  the  cistelliform  structure  is  acceler- 
ated to  the  immature  period,  and  in  Ismenia  (representing  an 
ismeniform  type  of  structure  in  the  higher  genera),  the  gwyni- 
form and  cistelliform  stages  are  larval,  and  the  platidiform 
represents  an  adolescent  condition.  Similar  comparisons  may 
be  made  in  the  other  genera.  Progressively  through  each 
series,  the  adult  structure  of  any  genus  forms  the  last  imma- 
ture stage  of  the  next  higher,  until  the  highest  member  in  its 
ontogeny  represents  serially,  in  its  stages  of  growth,  all  the 
adult  structures,  with  the  larval  and  immature  stages  of  the 


ONTOGENY  AND  PHYLOGENY  IN  BRACHIOPODA     289 

simpler  genera.  It  is  evident  that  in  the  identification  of 
species  belonging  to  the  Terebratellidse,  whether  recent  or 
fossil,  the  strict  specific  characters  must  be  given  first  con- 
sideration. Species,  therefore,  must  be  based  upon  surface 
ornaments,  form  and  color,  within  certain  limits,  and  genera 
only  upon  structural  features  developed  through  a  definite 
series  of  changes,  the  results  of  which  are  permanent  in 
individuals  evidently  fully  adult. 

In  each  line  of  progression  in  the  Terebratellidse,  the 
acceleration  of  the  period  of  reproduction,  by  the  influence  of 
environment,  threw  off  genera  which  did  not  go  through  the 
complete  series  of  metamorphoses,  but  are  otherwise  fully 
adult  and  even  may  show  reversional  tendencies  due  to  old 
age ;  so  that  nearly  every  stage  passed  through  by  the  higher 
genera  has  a  fixed  representative  in  a  lower  genus.  More- 
over the  lower  genera  are  not  merely  equivalent  to,  or  in 
exact  parallelism  with,  the  early  stages  of  the  higher,  but 
they  express  a  permanent  type  of  structure,  as  far  as  these 
genera  are  concerned,  and  after  reaching  maturity  do  not 
show  a  tendency  to  attain  higher  phases  of  development,  but 
thicken  the  shell  and  cardinal  process,  absorb  the  deltidial 
plates,  and  exhibit  all  the  evidences  of  senility. 


19 


3.    REVISION   OF   THE   FAMILIES   OF   LOOP- 
BEARING    BRACHIOPODA* 

(PLATES  XIV  and  XXIV) 

THE  recent  publications  of  Fischer  and  CEhlert,8'  9« 10>  f 
combined  with  previous  observations  by  Friele n  and  Des- 
longchamps,7  furnish  material  which  suggests  a  natural 
grouping  of  the  terebratuloids.  The  present  knowledge  is 
incomplete  in  some  details,  especially  as  regards  the  fossil 
genera,  yet  enough  is  available  to  simplify  the  arrangement 
of  the  leading  terebratuloid  types,  and  to  show  their  common 
relationships.  By  far  the  best  classifications  have  been  those 
proposed  by  Dall 3  in  1870,  and  by  Deslongchamps 7  in  1884. 
Only  in  the  light  of  recent  discoveries  is  it  possible  to  offer  a 
new  arrangement  of  the  genera. 

The  sub-order  Ancylobrachia,  proposed  by  Gray 12  in  1848, 
includes,  with  some  emendations,  all  the  genera  currently 
known  as  terebratuloids.  Taking  Gray's  name  for  the  entire 
group,  since  it  has  priority  over  Kampylopegmata,  Waagen,16 
1883,  it  is  found  to  comprise  two  distinct  types  of  brachial 
structure,  each  with  a  separate  genetic  history.  It  is  here 
proposed  to  recognize  these  two  types  as  of  family  impor- 
tance, according  to  the  interpretation  of  family  characters 
given  by  Agassiz.1 

The  TerebratulidoB. 

In  the  first  family,  the  Terebratulidse,  the  loop  is  always 
free  and  may  be  long  or  short.  It  is  developed  by  the  growth 

*  Trans.  Conn.  Acad.  ScL,  IX,  376-391,  395-398,  pis.  i,  ii,  1893. 

t  The  works  referred  to  by  numbers  are  cited  in  full  in  the  list  appended. 
An  excellent  summary  and  review  of  Fischer  and  (Ehlert's  papers,  8»  9.  10  by  Miss 
Agues  Crane,2  appeared  in  the  January  number  of  Natural  Science,  1893. 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA       291 

of  two  lamellae,  or  descending  branches,  from  the  points  of 
the  crura,  uniting  in  the  median  line.  The  central  portion 
may  be  narrow  or  medially  expanded.  In  some  genera,  re- 
curved ascending  branches  are  produced  by  the  partial  resorp- 
tion  of  the  broad  band  or  plate  forming  the  connection 
between  the  descending  branches.  The  cirri  in  early  stages 
of  the  animal  are  centrifugal  or  directed  outward.  The 
growth  of  the  loop  in  Terebratulina  has  been  illustrated  by 
Morse.15 

Terebratula  (Liothyrina)  and  Terebratulina  may  be  selected 
as  best  representing  the  Terebratulidse ;  for  Dyscolia,  Agul- 
hasia,  and  Eucalathis  do  not  represent  the  highest  develop- 
ment of  the  family  type,  but  must  be  regarded  as  degraded 
forms.  Among  fossil  genera  Cryptonella,  Megalanteris,  Die- 
lasma,  Centronella,  Rensselceria,  Stringocephalus,  and  some 
others,  probably  belong  here.  The  following  sub-families  can 
be  recognized :  (1)  the  Centronellinse,  (2)  Stringocephalinse, 
(3)  Terebratulinse,  and  (4)  Dyscoliinse.  The  adult  arm 
structure  in  Dyscolia  is  homologous  with  early  larval  features 
in  Terebratulina;  also  the  cirri  are  centrifugal  or  directed 
outward,  as  in  early  stages  of  Terebratulina^  and  not  cen- 
tripetal as  in  larval  Magellania. 

The  TerebratellidcB. 

The  loop  in  the  second  family,  for  which  the  name  Tere- 
bratellidse  is  retained,  undergoes  a  series  of  metamorphoses 
while  attached  to  a  dorsal  septum  during  the  larval  and  im- 
mature stages  of  the  animal,  and  in  the  higher  forms  results 
in  a  loop  of  secondary  growth  much  like  the  primary  loop 
of  some  of  the  early  genera  of  the  Terebratulidse.  The  cirri 
in  larval  stages  of  the  animal  are  centripetal  or  directed 
inwardly. 

In  one  division  of  the  Terebratellidse  the  stages  of  growth 
may  be  correlated  with  the  adult  loops  in  the  genera  Gwynia, 
Cistella,  Platidia,  Ismenia,  Muhlfeldtia,  Tercbratalia,*  and 
Dallina;^  while  in  another  division  a  quite  different  series 

*  Type  Terebratula  transversa  G.  B.  Sowerby. 
t  Type  Terebratula  septigera  Love'n. 


292  STUDIES  IN  EVOLUTION 

of  transformations  takes  place.  These  have  been  termed,  by 
Fischer  and  (Ehlert,8  the  prcemagadiform,  magadiform,  maga- 
selliform,  terebratelliform,  and  magellaniform  stages,  from 
their  resemblance  to  the  loops  of  the  genera  suggesting  these 
names.  The  prcemagadiform  stage  is  here  divided  into  the 
bouchardiform  and  megerliniform  stages.  To  these  may  be 
added  the  earlier  larval  stages  resembling  G-wynia  and  Cis- 
tella,  as  in  the  previous  group,  and  showing  a  parallel 
development  in  the  first  two  stages. 

These  two  groups  of  the  Terebratellidae  usually  have  been 
considered  as  part  of  the  family  Terebratulidae,  although 
King,14  in  1850,  proposed  the  name  Terebratellidae  to  include 
Terebratella,  Muhlfeldtia,  and  Ismenia,  on  account  of  the 
attachment  of  the  loop  to  the  septum  of  the  dorsal  valve  in 
these  genera.  Friele  n  and  Deslongchamps 7  next  showed  that 
Macandrevia  cranium  and  Dallina  septigera  passed  through  a 
series  of  changes  in  which  the  loop  was  united  to  a  septum  in 
all  but  the  last  stage.  This  completed  loop  in  Macandrevia, 
composed  of  two  descending  and  ascending  lamellae,  was  be- 
lieved to  be  homologous  with  the  loop  of  Terebratulina  and 
Liothyrina,  and  the  family  proposed  by  King  fell  into  disuse. 
It  can  now  be  shown,  however,  that  the  loop  of  Macandrevia 
is  made  up  of  a  primary  portion  corresponding  to  the  entire 
loop  of  Liothyrina,  and  a  secondary  part  which  has  no  equiv- 
alent in  the  calcified  lamellae  of  Liothyrina  or  Terebratulina, 
but  in  them  is  represented  in  the  fleshy  portion  of  the  arms, 
as  previously  recognized  by  Hancock.13 

The  loop  in  Terebratulina  is  equivalent  to  the  descending 
lamellae  in  Terebratella,  from  the  crural  points  down  to  and 
including  the  bands  connecting  with  the  septum.  In  Magel- 
lania  and  Macandrevia  the  connecting  bands  of  Terebratella 
are  represented,  except  in  old  specimens,  by  slight  projec- 
tions from  the  descending  branches,  and  in  these  genera, 
therefore,  the  primary  loop  is  incomplete.*  The  true  rela- 

*  The  prongs  or  points  below  the  ends  of  the  crura  on  the  primary  lamellae 
in  Spirifer  also  represent  portions  of  a  loop.  More  close  analogy  is  seen  in  later 
forms  of  Atrypa  having  a  disunited  loop. 


FAMILIES  OF  LOOP-BEARING  BRACHIOPODA        293 

tions  and  homologies  of  these  parts  can  best  be  shown  in  a 
series  of  figures. 

Plate  XIV,  figures  Ci,  Di,  represent  the  loop  in  a  young 
Macandrevia  cranium  in  the  so-called  platidiform  stage,  show- 
ing a  complete  primary  loop  and  the  beginning  of  a  secondary 
loop  in  the  middle,  on  top  of  the  septum.  A  later  stage  of 
the  same  species  (Plate  XIV,  figure  GI)  has  the  structure 
of  Terebratalia.  The  descending  lamellae  and  the  median 
V-shaped  plate  correspond  to  the  primary  loop,  while  the 
secondary  loop  or  posteriorly  recurved  portion  has  greatly 
increased  in  size.  A  later  stage,  nearly  complete  (Plate 
XXIV,  figure  1),  shows  two  points  (p)  on  the  descending 
lamellae,  which  are  remnants  of  the  connecting  band  in  pre- 
vious stages.  The  parts  homologous  with  the  loop  of  the 
first  stage  and  with  the  loop  of  Terebratulina  are  shaded. 
Greater  emphasis  is  expressed  by  figures  2,  3,  Plate  XXIV, 
where  the  cirrated  brachia  and  calcareous  supports  are  both 
represented  in  the  genera  Terebratulina  and  Magellania.  It 
is  readily  seen  that  the  arm  structure  is  the  same  in  both,  but 
that  the  calcareous  loops  which  are  darkly  shaded  are  very 
different  in  form. 

The  family  Terebratellidae  should  therefore  be  reinstated 
on  the  evidence  here  given.  The  development  of  Terebra- 
tella  may  be  reviewed  for  the  leading  characteristics  of  one 
division  of  the  family.  The  type  is  Terebratella  chiliensis 
Broderip,  sp.  =  T.  dorsata  Gmelin,  sp.,  from  the  Straits  of 
Magellan.  Fischer  and  CEhlert8  have  described  in  detail  the 
development  of  the  loop  in  this  form.  Their  researches  also 
include  Magellania  venosa  Solander,  sp.,  which  was  found  to 
pass  through  all  the  stages  of  Terebratella  dorsata,  and  after 
losing  the  processes  connecting  the  primary  lamellae  with  the 
septum  finally  results  in  adult  Magellania. 

MagellaniincB. 

The  first  stage  described  by  these  authors  (Plate  XIV, 
figure  B)  showed  only  a  septum  anterior  to  the  middle  of  the 


294  STUDIES  IN  EVOLUTION 

dorsal  valve.*  The  next  stage  was  called  the  prcemagadiform 
stage  (Plate  XIV,  figures  Ca,  -Da),  but  it  may  well  be  divided 
into  two  stages,  which  correspond  in  structure  to  adult 
Bouchardia  and  Megerlina.  The  bouchardiform  stage  (figure 
Ca)  has  a  high  quadrangular  septum  in  the  dorsal  valve,  and 
on  the  posterior  distal  angle  there  is  a  small  circle,  or  calca- 
reous ring.  The  crura  are  present,  but  the  primary  lamellae 
have  not  yet  appeared.  In  the  next  stage,  the  megerliniform 
(figures  Da,  Da'),  the  ring  has  increased  in  size,  and  below, 
on  the  septum,  have  appeared  two  projections  or  points, 
which  are  the  beginnings  of  the  descending  primary  branches. 

The  subsequent,  or  magadiform,  stage  (Plate  XIV,  figure 
Ea)  shows  the  completion  of  the  descending  branches  to  form 
the  primary  loop,  and  also  the  enlargement  of  the  secondary 
loop  or  ring.  During  further  growth  the  primary  and 
secondary  loops  approach  each  other  on  the  septum,  then 
coalesce  and  make  the  magaselliform  condition  represented  in 
figure  Fa. 

The  ventrally  projecting,  free  portion  of  the  septum  next 
is  absorbed,  and  the  branches  of  the  loop  become  attenuated, 
but  still  the  descending  branches  remain  connected  with  the 
septum,  and  thus  the  terebratelliform  stage  is  completed 
(Plate  XIV,  figure  Ga). 

Magellania  venosa,  after  passing  through  all  the  stages 
described,  including  the  terebratelliform,  loses  the  connecting 
bands,  and  develops  into  the  final  magellaniform  type  of 
structure  (Plate  XIV,  figure  Ha).  Moreover,  Magellania 
lenticularis,  M.  flavescens,  Terebratella  cruenta,  and  T.  rubi- 
cunda,  as  far  as  observed,  correspond  closely  in  their  develop- 
ment with  the  morphogeny  of  M.  venosa. 

A  fact  of  importance  noticed  by  Fischer  and  CEhlert8  is 
that  these  species  are  confined  to  the  southern  hemisphere. 
The  other  austral  types  of  terebratuloids,  exclusive  of  the 
genera  of  Terebratulidas,  as  here  restricted,  are  Magasella 
(M.  Cumingi),  Kraussina,  Megerlina,  and  Bouchardia.  In 

*  An  earlier  gwyniform  stage  has  been  observed  by  the  writer  in  a  young 
example  of  Magellania  Jlavescens. 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA      295 

their  brachial  supports  these  all  approximate  early  stages  of 
the  higher  genera  Magellania  and  Terebratella.  They  must 
be  regarded  as  arrested  and  degraded  forms. 

The  brachial  supports  in  Kraussina  and  Bouchardia  are 
merely  portions  of  the  ascending  branches,  or  secondary  loop, 
on  the  septum,  without  any  traces  of  the  descending  branches, 
or  primary  lamellae.  These  genera  may  be  compared  with 
the  bouchardiform  stage  of  Terebratella  dorsata.  One  grade 
higher  is  exhibited  in  Megerlina  (type  M.  Lamarckiana  David- 
son) in  which  there  is  added  to  the  Kraussina  structure  two 
processes  apparently  homologous  with  the  points  belonging 
to  the  descending  branches  appearing  on  the  septum  in  the 
megerliniform  stage  of  T.  dorsata.  These  atavistic  genera 
are  all  austral  in  their  distribution,  but  not  strictly  polar, 
occurring  as  they  do  off  the  coasts  of  South  Africa,  Brazil, 
Australia,  St.  Paul's  Island,  etc. 

In  reviewing  this  group  of  genera,  it  is  seen  that  the  high- 
est member  of  the  series  is  Magellania,  which  reaches  its 
maximum  development  in  size  and  number  of  species  in 
antarctic  seas.  The  next  genus  below,  Terebratella,  ranges 
still  further  toward  the  equator,  while  the  atavistic  types 
Kraussina,  Megerlina,  and  Bouchardia  do  not  occur  in  polar 
regions,  but  are  nevertheless  austral  in  their  distribution. 

Dallinince. 

The  northern  hemisphere  furnishes  a  series  of  genera  and 
species,  which,  passing  through  a  different  and  distinct  series 
of  loop  metamorphoses,  attains  in  the  higher  members  the 
same  result  as  those  of  the  southern  fauna,  constituting  a 
case  of  exact  parallel  development.  Thus  the  northern  Ma- 
candrevia  cranium,  Dallina  septigera,  D.  Raphaelis,  D.  G-rayi, 
Terebratalia  transversa,  T.  coreanica,  T.  spitzbergensis,  and 
T.  frontalis  are  very  similar  in  the  adult  characters  of  the 
loop  to  the  southern  Magellania  venosa,  M.  kerguelenensis, 
M.  Wyvillii,  M.  flavescens,  M.  lenticularis,  Terebratella  dor- 
sata, T.  cruenta,  and  T.  rubicunda.  It  is  only  when  their 
development  is  examined  that  a  difference  is  manifest. 


296  STUDIES  IN  EVOLUTION 

By  observing  the  stages  of  development  in  the  austral  and 
boreal  terebratellids,  it  is  seen  that  both  start  from  a  common 
larval  stage,  and  divergence  into  two  lines  begins  in  the  first 
adolescent  stages,  so  that  the  series  of  metamorphoses  in 
each  is  quite  distinct  nearly  to  the  end.  This  in  itself  might 
not  require  that  the  austral  and  boreal  species  should  be 
referred  to  different  genera  and  placed  in  different  sub-fam- 
ilies ;  but  when  it  is  found  that  all  the  other  southern  genera 
of  the  Terebratellidse  represent  arrested  and  degraded  stages 
in  the  development  of  a  southern  Terebratella  or  Magellania, 
and  that  the  northern  genera  represent  similar  stages  in  the 
development  of  a  northern  high  type,  such  a  separation  neces- 
sarily follows.*  Moreover,  these  stages  have  a  more  profound 
significance,  as  several  of  them  in  both  regions  represent 
established  genera  now  extinct. 

A  feature  which  may  be  of  service  in  distinguishing  adult 
recent  shells  is,  that  the  Dallininas  have  small  cardinal  proc- 
esses, and  the  interior  of  the  dorsal  beak  is  usually  grooved 
to  the  apex,  while  in  Magellaniinse  there  is  a  well-developed 
projecting  cardinal  process  often  filling  the  cavity  of  the 
beak.  The  lower  genera  can  be  readily  determined  by  the 
characters  of  the  loop  and  by  the  median  septum,  which  is 
generally  low  in  the  Dallininse  and  projecting  above  the  loop 
in  the  Magellaniinse.  With  these  considerations  in  mind,  the 
metamorphoses  and  relations  of  the  northern  Terebratellidse 
may  be  described. 

There  are  two  finished  types  of  northern  genera,  which  are 

*  Platidia  seems  to  be  an  exception  in  the  distribution  of  the  northern  genera, 
as  it  has  been  recorded  from  Marion  Island,  in  the  southern  Indian  Ocean.  The 
northern  forms  referred  to  Mayasella  are  without  the  characteristic  high  septum 
of  M.  Cumingi,  and  appear  to  be  stages  of  development  of  a  higher  northern 
form. 

In  Fischer's  "Manuel  de  Conchyliologie,"  p.  1246,  CEhlert,iu  discussing  the 
geographical  distribution  of  brachiopods,  says  :  "  Parmi  les  Brachiopods  il  en  est, 
dont  la  distribution  est  en  rapport  avec  la  temperature  re"gionale ;  c'est  ainsi 
qu'un  certain  nombre  d'especes  sont  particulieres  aux  mers  qui  avoisinent  les 
poles,  chaque  hemisphere  ayant  ses  formes  speciales  qui  lui  appartiennent  en 
propre,  a  1'exception  de  Terebratulina  caput-serpentis,  var.  septentrional  is,  qui  se 
trouve  k  la  fois  dans  1'hemisphere  austral  et  dans  I'hemSsphere  bore'al." 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA       297 

taken  as  characteristic  examples.  One  is  the  Macandrevia 
cranium  Miiller,  and  the  other  has  been  called  Magellania 
(Waldheimia)  septigera  Love*n.  In  the  light  of  the  geo- 
graphic, genetic,  and  ontogenetic  facts,  the  application  of 
the  law  of  morphogenesis  necessitates  a  new  generic  name 
for  the  second.  Magellania  cannot  be  retained,  as  the 
type  is  M.  venosa  from  Tierra  del  Fuego,  and  therefore 
belongs  to  the  southern  line  having  a  different  series  of 
metamorphoses.  Neither  can  it  be  referred  to  Macandrevia, 
on  account  of  its  well-developed  septum  at  maturity ;  nor  to 
Endesia  (type  E.  cardium  Lamarck,  from  the  Jurassic),  since 
that  genus  has  strong  dental  plates  in  the  ventral  valve,  divid- 
ing the  cavity  of  the  beak  into  three  chambers.  Waagen  16 
shows  that  these  features  —  septal  and  dental  plates  —  are 
entitled,  in  the  terebratuloids,  to  rank  as  generic  characters. 

The  name  Dallina,  nov.  gen.,  is  therefore  proposed,  to 
include  shells  of  the  type  of  Terebratula  septigera  Love*n;  as 
Dallina  Raphaelis  Ball,  sp.,  D.  Crrayi  Davidson,  sp.,  and 
D.  floridana  Pourtales,  sp.  The  genus  is  given  in  honor  of 
William  H.  Dall,  whose  name  has  long  been  intimately  asso- 
ciated with  the  best  work  on  recent  Brachiopoda. 

There  still  remain  the  northern  species  heretofore  referred 
to  Terebratella,  which  differ  from  true  Terebraiella  (type 
T.  dorsata)  in  the  same  manner  and  degree  as  Dallina  from 
Magellania.  These  also  require  a  special  designation,  and 
the  name  Terebratalia,  nov.  gen.,  is  proposed,  based  on  Tere- 
bratula transversa  G.  B.  Sowerby,  as  the  type. 

The  earliest  stages  of  development  in  the  Dallina  and 
Terebratalia  branch  of  the  Terebratellidse  have  been  observed 
by  the  writer  in  T.  transversa  Sowerby  and  T.  obsoleta  Dall.4  * 
They  represent  first  a  shell  without  a  septum  in  the  dorsal 
valve,  and  without  calcified  supports  to  the  brachia  (Plate 
XIV,  figure  A).  The  structure  just  before  the  appearance 

*  Originally  described  as  Terebratella  occidentalis,  var.  obsoleta,  by  Dall,  but 
now  considered  by  him  as  a  distinct  species.  The  complete  development  of  the 
brachial  supports  in  this  species  is  shown  in  paper  No.  8  of  this  series. 


298  STUDIES  IN  EVOLUTION 

of  the  septum  is  the  same  as  that  described  in  G-wynia  by 
King.  The  brachia  form  a  slender  fleshy  ellipse  or  circle, 
resting  in  front  on  the  floor  of  the  interior  of  the  dorsal 
valve,  with  the  tentacles  or  cirri  centripetal  or  directed 
inward,  as  in  an  early  stage  of  Cistella.  After  this  gwyni- 
form  stage  the  growth  of  the  septum  inflects  the  circlet  of 
tentacles,  producing  a  condition  identical  with  that  in  adult 
Cistella  (Plate  XIV,  figure  B).  It  is  therefore  called  the 
cistelliform  stage. 

The  succeeding  transformations  in  Dallina  septigera  and 
Macandrevia  cranium  have  been  fully  described  by  Friele.11 
These  species,  with  Terebratalia  obsoleta  Dall,  sp.,  make  three 
typical  northern  forms  whose  development  has  been  observed. 
They  agree  in  every  essential  detail,  and  may  be  described 
in  general  terms.  The  first  stage  described  by  Friele  (Plate 
XIV,  figures  Ci,  Di),  showed  the  growth  of  the  descending 
lamellae,  their  union  with  the  septum,  and  the  appearance  of 
a  small  ring  on  the  top  of  the  septum,  which  is  the  beginning 
of  the  ascending  branches,  or  secondary  loop.  This  condi- 
tion was  correlated  with  the  genus  Platidia,  by  Deslong- 
champs,7  and  was  called  the  platidiform  stage.  It  has  also 
been  called  the  centronelliform  stage  by  Fischer  and  QEhlert,8 
but,  as  Centronella  is  not  known  to  have  a  septum  supporting 
the  loop,  the  name  is  not  adopted  here. 

The  lower  anterior  part  of  the  secondary  loop  begins  to 
divide  very  early  (Plate  XIV,  figure  Di),  and,  at  the  same 
time,  the  ends  of  the  descending  branches  broaden  and 
approach  the  top  of  the  septum,  being  thus  in  juxtaposition 
to  the  ascending  branches  (as  in  figure  Ei),  called  the  ismeni- 
form  stage.  Lacunse  are  then  produced  by  resorption  in  the 
broad  plates  forming  the  ascending  branches,  and  the  struc- 
ture of  the  supports  at  this  time  (figure  Fi),  resembles  that 
in  adult  Muhlfeldtia  sanguinea  and  M.  truncata  (figure  F3), 
in  which  the  secondary  loop  is  still  attached  to  the  septum. 
This  stage  is  here  termed  the  muhlfeldtiform  stage.  A  further 
broadening  of  the  loop  and  completion  of  the  structures 
already  outlined,  with  the  recession  of  the  secondary  connect- 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA       299 

ing  bands  from  the  septum,  result  in  Laqueus  (figures  G4, 
Gs),  which  has  connecting  bands  from  the  ascending  to  the 
descending  lamellae  and  from  the  latter  to  the  septum.* 

The  absorption  of  the  connecting  bands  from  the  ascend- 
ing branches  completes  the  Terebratalia  stage  (Plate  XIV, 
figures  Gi,  Ga),  in  Macandrevia  and  Dallina,  and  is  the  adult 
condition  in  Terebratalia  transversa^  T.  obsoleta,  T.  frontalis, 
and  T.  coreanica  (figure  G3).f 

Finally,  the  resorption  of  the  connecting  bands  from  the 
descending  branches  produces  the  Dallina  structure,  and  the 
further  resorption  of  the  septum  terminates  the  series  in 
Macandrevia  (Plate  XIV,  figure  Hi). 


Comparisons  and  Homologies. 

Thus  the  genera  of  the  Terebratellidse  begin  their  larval 
development  in  a  form  like  Gwynia,  having  no  calcified 
brachial  supports,  and  with  a  simple  circle  of  centripetally 
directed  tentacles.  Then  by  the  growth  of  a  septum  in  the 
middle  of  the  dorsal  valve,  a  cistelliform  stage  is  reached. 
From  this  point  divergence  begins,  and  there  is  one  series 
of  transformations  resulting  in  Macandrevia,  and  another 
terminating  in  Magellania,  the  mature  loops  in  both  groups 
being  practically  alike.  Macandrevia  and  Dallina  are  mor- 
phically  equivalent  to  MageUania,  and  Terebratalia  is  also  in 
exact  parallelism  with  Terebratella. 

A  more  graphic  presentation  of  the  development  and  rela- 
tions of  the  genera  is  shown  on  Plate  XIV,  in  which  the  stages 
of  growth  of  Magellania  and  Macandrevia  are  represented 
on  two  ontogenetic  lines.  Outside  are  placed  other  species 
and  genera,  with  their  known  stages  of  growth  so  arranged 

*  The  name  Megerlina  Jeffrey  si  was  given  to  a  stage  of  Laqueus  californica 
from  its  having  a  structure  like  Megerlina  (=  Muhlfeldtid)  truncata,  thus  indi- 
cating clearly  the  close  relationship  of  these  genera. 

t  T.  spitzbergensis  and  T.  Marice,  from  the  unfinished  appearance  of  their 
brachial  supports,  possibly  will  be  found  to  belong  to  a  higher  member  of  the 
series ;  for  example,  Dallina. 


300  STUDIES  IN  EVOLUTION 

that  their  equivalent  stages  fall  into  parallel  lines  with 
those  of  Magellania  and  Macandrevia* 

The  line  begins  in  an  early  larval  stage  (Plate  XIV, 
figure  A),  in  which  there  is  a  simple  circlet  of  tentacles 
without  a  calcined  loop,  a  structure  comparable  in  every 
respect  with  Crwynia  (figure  Aa). 

The  next  stage  (Plate  XIV,  figure  B)  shows  the  growth  of 
a  septum  inflecting  the  line  of  tentacles,  and  producing  an 
arrangement  of  parts  similar  to  Cistella,  although  the  loop  is 
not  calcined.  The  completion  of  this  structure  results  in 
Cistella  (figure  Bi),  and  specimens  having  the  characters 
presented  by  figures  B,  Bi,  are  referred  to  the  cistelliform 
stage.  A  fossil  representative  of  this  type  is  Zellania,  from 
the  Jurassic  (figure  Ba). 

Megathyris  (figure  B2)  offers  nearly  the  same  structure  as 
Cistella,  but  the  growth  of  two  lateral  septa  or  projections 
has  produced  two  additional  inflections  in  the  loop.  This 
completes  the  line  of  development  in  the  MegathyrinsD. 

Next,  considering  the  Dallininse  in  their  ontogeny  and 
morphology,  it  is  found  that  after  passing  through  the 
gwyniform  and  cistelliform  stages  (Plate  XIV,  figures  A,  B)  a 
form  like  Platidia  is  reached  (figure  Ci),  of  which  Platidia  is 
the  living  adult  representative  (figure  O).  The  platidiform 
stage  is  shown  in  Macandrevia  cranium  (figures  Ci,  Di) ; 
Dallina  septigera  (figure  D2) ;  D.  floridana  (figure  C3) ;  and 
Muhlfeldtia  sanguinea  (figures  €2,  Ds). 

The  growth  of  the  secondary  loop  on  the  septum  and  the 
subsequent  partial  resorption  produces  a  structure  (1)  like 
that  in  the  fossil  genus  Ismenia,  and  (2)  one  identical  with 
that  of  adult  Muhlfeldtia  (^M.  truncata  and  M.  sanguinea). 
In  Plate  XIV,  figures  Ei-E5  show  the  ismeniform  stage  of 
Macandrevia,  Dallina,  and  Muhlfeldtia,  as  well  as  the  final  con- 
dition of  Ismenia,^  and  figures  Fi-F4  represent  the  muhlfeldti- 

*  The  illustrations  on  Plate  XIV  are  taken  from  Davidson,5-6  Fischer  and 
CEhlert,8  Deslongchamps,7  and  Friele,11  with  original  drawings  by  the  writer. 

t  Figures  Fs  and  F6,  Plate  XIV,  are  from  Davidson.5  They  are  Jurassic 
species,  and  were  referred  to  Terebratella  (T.  furcata  Sowerby,  figure  Fs,  and 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA      301 

form  stage  of  Macandrevia,  Dallina,  Laqueus,  and  the  adult 
structure  of  Muhlfeldtia  sanguined.  After  this,  the  union  of 
the  primary  and  secondary  loops  and  their  removal  from  the 
septum  to  which  they  remain  attached  only  by  connecting 
processes  form  a  structure  like  that  in  Laqueus  (figures  G*, 
Gs),  and  the  resorption  of  the  connecting  bands  from  the 
ascending  branches  of  the  loop  completes  the  terebrataliform 
stage  of  Macandrevia  and  Dallina,  as  shown  in  Plate  XIV, 
figures  Gi,  G2.  Terebratalia  is  the  present  fixed  genus  of 
this  type  of  structure  (figure  Gs),  and  Trigonosemus  (figure 
G5),  is  a  Cretaceous  representative.  Finally,  by  the  resorp- 
tion of  the  bands  of  the  terebrataliform  stage,  the  structure 
of  the  highest  genera,  Macandrevia  and  Dallina,  is  reached 
(figures  Hi-H5). 

The  first  stage  after  the  cistelliform  in  the  Magellan iinse, 
the  austral  branch  of  the  Terebratellidse,  is  represented  for 
Terebratella  dorsata,  in  Plate  XIY,  figure  Ca.  Kraussina 
(figures  Cb,  Cc)  has  a  simple  fork  or  V-shaped  process  on  the 
septum,  which  apparently  represents  an  incomplete  secondary 
loop.  The  relations  of  Bouchardia  (figure  Ccf)  to  this 
bouchardiform  stage  of  Terebratella  are  more  evident.  After 
this  stage  the  beginnings  of  the  primary  loop,  or  descending 
branches,  appear  as  two  projections  on  each  side  of  the 
septum  (figure  Da').  Megerlina  (figure  D5)  shows  this  ad- 
vance over  Kraussina. 

The  completion  of  the  descending  branches  in  the  next,  or 
magadiform,  stage  is  represented  for  Terebratella  dorsata,  in 
Plate  XIV,  figure  Ea  ;  T.  cruenta,  figure  EC  ;  T.  rubicunda, 
figure  E<# ;  Neothyris  lenticularis,  figure  E£>  /  Magasella  Cum- 
ingi,  figure  E/.  The  Cretaceous  equivalent,  Magas,  is  shown 
in  figure  Ee.  In  all  these  forms  the  septum  projects  above 
the  descending  lamellae  nearly  to  the  ventral  valve. 

T.  Buckmani  Moore,  figure  Fe).  A  strict  interpretation  of  that  genus  based  upon 
T.  dorsata,  the  type,  excludes  these  species,  which  agree  with  the  definition  of 
Ismcnia  in  that  the  ascending  and  descending  branches  are  attached  directly  to 
the  septum.  They  may  be,  however,  stages  of  growth  of  higher  forms. 


302 


STUDIES  IN  EVOLUTION 


After  the  magadiform  stage  the  descending  and  ascend- 
ing branches  approach  and  unite,  and  at  the  same  time 
there  is  a  narrowing  of  the  latter  (Plate  XIV,  figures  Fa-Fe). 
Magasella  Cumingi  Davidson  seems  to  be  the  only  permanent 
adult  representative  of  this  structure  which  has  yet  been 
found. 

The  further  narrowing  of  the  lamellae,  broadening  of  the 
loop,  and  absorption  of  the  free  portion  of  the  septum,  result 
in  the  terebratelliform  structure  (Plate  XIV,  figures  Ga-Gd), 
comparable  directly  with  figures  Gi-G6  of  the  Dallininae  or 
boreal  genera.  Also,  as  in  the  Dallininse,  the  disappearance 
of  the  connecting  bands  completes  the  magellaniform  stage, 
and  terminates  the  series  (figures  Ha-He). 

The  stages  of  growth  of  the  genera  belonging  to  the  three 
sub-families  of  the  Terebratellidse,  —  the  Megathyrinse,  Dal- 
lininse,  and  Magellaniinse,  are  further  correlated  in  the  accom- 
panying tables.  It  must  be  understood,  of  course,  that  the 
larval  and  immature  stages  have  not  been  observed  in  all  the 
genera,  but  from  the  known  ontogeny  of  several  of  the  lower 
and  higher  forms,  and  from  evident  homologies  of  structure, 
such  stages  may  be  inferred. 

Morphogeny  from  Gwynia  to  Megathyms. 


Periods. 

Stages. 

Stages. 

Stages. 

Larval 
Adolescent 
Mature 

gwyniform'? 
gwyniform 
Gwynia 

gwyniform 
cistelliform 
Cistella 

gwyniform 
cistelliform 
Megathyris 

The  simplest  genus,  Gwynia,  as  far  as  known,  passes 
through  no  metamorphoses,  and  has  the  same  structure 
throughout  the  adolescent  period,  up  to  and  including  the 
mature  condition.  In  the  ontogeny  of  Cistella  the  gwyni- 
form stage  through  acceleration  has  become  a  larval  condi- 
tion. In  Platidia  the  cistelliform,  structure  is  accelerated 
to  the  immature  period,  and  in  Ismenia  (representing  an 
ismeniform  type  of  structure  in  the  higher  genera),  the  gwyni- 


FAMILIES   OF  LOOP-BEARING  BRACHIOPODA         303 


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304  STUDIES  IN  EVOLUTION 

form  and  cistelliform  stages  are  larval,  and  the  platidiform 
represents  an  adolescent  condition.  Similar  comparisons  may 
be  made  in  the  other  genera.  Progressively  through  each 
series  the  adult  structure  of  any  genus  forms  the  last  imma- 
ture stage  of  the  next  higher,  until  the  highest  member  in 
its  ontogeny  represents  serially,  in  its  stages  of  growth,  all 
the  adult  structures,  with  the  larval  and  immature  stages  of 
the  simpler  genera. 

Conclusions. 

It  is  impossible,  with  present  knowledge,  to  go  deeply  into 
the  chronological  history  of  the  genera  of  the  Terebratellidse. 
They  certainly  appeared  before  Jurassic  time,  because  they 
were  then  well  represented  by  several  characteristic  genera; 
namely,  Kingena,  Ismenia,  Zellania,  and  Megathyris,  al- 
though some  of  them  may  represent  incomplete  develop- 
ments of  higher  forms.  Also,  as  among  recent  species, 
several  separate  generic  and  specific  names  may  have  been 
given  to  stages  of  growth  of  a  few  species.  It  is  evident  that, 
in  the  identification  of  specimens  in  this  family,  whether  re- 
cent or  fossil,  the  strict  specific  characters  must  be  given  first 
consideration.  Species,  therefore,  must  be  based  upon  surface 
ornaments,  form,  and  color,  within  certain  limits,  and  genera 
only  upon  structural  features  developed  through  a  definite 
series  of  changes,  the  results  of  which  are  permanent  in  indi- 
viduals evidently  fully  adult. 

The  austral  distribution  of  the  Magellaniinse  and  the  boreal 
distribution  of  the  Dallininsa  have  been  emphasized  already, 
but,  as  has  been  stated,  this  difference  in  geographical  posi- 
tion in  itself  does  not  necessarily  constitute  a  basis  for  sub- 
family separation.  The  facts  of  development,  however,  and 
the  distribution  of  the  genera  show  that  the  radical  stock 
was  dispersed  probably  in  Mesozoic  time,  and  since  then 
evolution  has  gone  forward  through  different  lines  of  progres- 
sion, and  has  terminated  in  similar  types  of  structure. 

In  each  line  of  progression  the  acceleration  of  the  period 


FAMILIES  OF  LOOP-BEARING  BRACHIOPODA      305 

of  reproduction,  by  the  influence  of  environment,  threw  off 
genera  which  did  not  go  through  the  complete  series  of  meta- 
morphoses, but  are  otherwise  fully  adult,  and  even  may  show 
reversional  tendencies  due  to  old  age ;  so  that  nearly  every 
stage  passed  through  by  the  higher  genera  has  a  fixed  repre- 
sentative in  a  lower  genus.  Moreover,  the  lower  genera  are 
not  merely  equivalent  to  or  in  exact  parallelism  with  the 
early  stages  of  the  higher,  but  they  express  a  permanent  type 
of  structure,  as  far  as  these  genera  are  concerned,  and  after 
reaching  maturity  do  not  show  a  tendency  to  attain  higher 
phases  of  development,  but  thicken  the  shell  and  cardinal 
process,  absorb  the  deltidial  plates,  and  exhibit  all  the  evi- 
dences of  senility  and  reversion  presented  during  the  old  age 
of  the  higher  genera.  Kraussina  shows  a  partial  loss  of  del- 
tidial plates  and  a  thickened  septum,  with  two  strong  prongs 
representing  the  ascending  branches,  which  in  other  genera 
are  very  delicate.  Bouchardia,  Magasella  (M.  Cumingi),  and 
Agulhasia  have  excessively  thickened  hinge-plates  and  mus- 
cular fulcra ;  the  deltidial  plates  are  obsolete,  but  the  pedicle 
is  enclosed  by  the  growth  and  overlapping  of  the  edges  of 
the  delthyrium.  Megathyris  and  Cistella  deposit  lime  on  the 
interior  of  the  valves,  often  in  the  form  of  nodes  and  ridges, 
while  the  pedicle  in  its  growth  encroaches  upon  the  beaks  of 
both  valves  to  the  final  elimination  of  the  deltidial  plates. 
Platidia  represents  the  extreme  of  this  encroachment.  Thus 
there  are  incomplete  types  of  brachial  development  in  these 
genera,  accompanied  by  positive  evidences  of  senility  and 
retrogression  in  other  shell  characters. 

Classification. 

In  the  light  of  the  previous  discussion,  the  classification  of 
the  families  and  genera  of  Ancylobrachia,  with  the  exception 
of  the  fossil  forms,  is  comparatively  simple.  Among  the 
extinct  genera  of  the  TerebratulidsB,  or  the  family  in  which 
the  loop  is  a  development  of  the  descending  branches,  Cen~ 
tronella  and  Eensselceria  belong  to  one  sub-family;  Stringo- 
cephalus  to  another;  and  Megalanteris,  Cryptonella,  Dielasma^ 

DictyothyriS)  and  others,  will  come  in  the  Terebratulinse. 

20 


306  STUDIES  IN  EVOLUTION 

As  for  the  long-looped  Mesozoic  forms,  it  is  evident  that 
many  of  them  underwent  metamorphoses  in  their  develop- 
ment while  attached  to  a  dorsal  septum.  The  presence  with 
them  of  such  genera  as  Kingena,  Ismenia^  and  Muhlfeldtia, 
points  to  their  intimate  relations  with  the  boreal  stock  of  the 
Terebratellidae,  and  therefore  indicates  that  the  Mesozoic 
species  with  long  recurved  loops  will  in  all  probability  be 
found  to  agree  with  Macandrevia,  Eudesia,  Dallina^  Trigo- 
nosemus,  Lyra,  Terebratalia,  Laqueus,  Kingena,  Ismenia,  and 
Milhlfeldtia.  Magas  is  exceptional,  as  it  belongs  to  the 
austral  group  of  genera,  and  suggests  the  likelihood  that 
some  of  the  higher  species  and  genera  associated  with  it 
geologically  may  prove  to  belong  to  the  same  branch. 

The  septum  in  the  Terebra  tell  idee  appears  before  the 
branches  of  the  loop,  and  is  an  important  character  in  the 
series  of  metamorphoses  until  the  last  stage.  At  first  it  is 
merely  a  support  to  the  introverted  growing  cirrated  portion 
of  the  lophophore.  This  elongation  of  the  cirrated  margin 
next  results  in  an  arch  on  the  septum  in  the  platidiform  and 
ismeniform  stages.  Further  elongation  produces  the  median 
coiled  arm,  and  the  structures  impeding  its  growth  become 
resorbed,  so  that  the  transverse  space  between  the  lamellae 
widens,  the  connecting  bands  and  high  septum  recede,  and 
finally  even  these  disappear  in  Macandrevia  and  Magellania. 

The  following  table  represents  the  genera  arranged  accord- 
ing to  the  views  expressed  in  this  paper :  — 

Family  TEREBRATULID^E  Gray. 

Loop  free,  developed  by  the  growth  and  modification  of 
two  primary  lamellae.  Cirri  directed  outward  in  larval 


CENTRONELLIN^:  Waagen. 

Loop  composed  of  two  descending  lamellae  uniting  in  the 
median  line,  forming  a  broad  arched  plate. 

Centronella  Billings.  Juvavella  Bittner. 

Rensselceria  Hall.  Nucleatula  Zugmayer. 

Newberria  Hall. 


FAMILIES  OF  LOOP-BEARING  BRACHIOPODA      307 


STRINGOCEPHALIN.E  Dall. 

Loop  long,  following  the  margin  of  the  dorsal  valve,  not 
recurved  in  front.     Probably  no  median  coiled  arm. 

Stringocephalus  Defrance. 


TEREBRATULIN^E   Dall. 

Loop  usually  short.  Ascending  branches,  when  present, 
formed  by  partial  resorption  of  a  primary  centronelliform  loop. 
A  median  unpaired  coiled  arm  exists  in  recent  genera. 

Cryptonella  Hall.  Terebratulina  d'Orbigny. 

Megalanteris  Suess.  Dictyothyris  Douville. 

Dielasma  King.  Pygope  Link. 

Dielasmina  Waagen.  Goenothyris  Douville. 

Liothyrina  (Ehlert.  G-lossothyris  Douville. 

DYSCOLIINJE  (=  Dyscoliidse  Fischer  and  (Ehlert  emend.). 

t 
Loop  short,  continuous  with  the  cirrated  edge  of  the  lopho- 

phore.     No  coiled  median  arm. 

Dyscolia  Fischer  and  (Ehlert.  ?  Agulhasia  King. 

Eucalathis  Fischer  and  (Ehlert. 

Family  TEREBRATELLID.E  King  emend. 

Loop  in  the  higher  genera  composed  of  two  primary  and 
two  secondary  lamellae,  development  indirect,  passing  through 
a  series  of  distinct  metamorphoses  while  attached  to  a  dorsal 
septum.  Cirri  directed  inwardly  during  larval  stages,  at 
which  time  the  structure  is  comparable  to  that  of  Grwynia 
and  Cislella. 

DALLININ^,  n.  sub-fam. 

Loop  composed  of  descending  and  ascending  lamellae,  pass- 
ing in  the  highest  genera  through  metamorphoses  comparable 
to  the  adult  structure  of  Platidia,  Ismenia,  Muhlfeldtia, 


808  STUDIES  IN  EVOLUTION 

Terebratalia,  and  Dallina.  The  lower  genera,  therefore,  do 
not  progress  to  the  final  stages. 

Macandrevia  King.  Laqueus  Dall. 

Dallina  Beecher.  Muhlfeldtia  Bayle. 

Eudesia  King.  Kingena  Davidson. 

Terebratalia  Beecher.  Ismenia  King. 

Trigonosemus  Koenig.  Platidia  Costa. 

Lyra  Cumberland. 

MAGELLANIIN.E,   n.  sub-fam. 

Loop  composed  of  descending  and  ascending  branches,  pass- 
ing in  the  higher  genera  through  metamorphoses  comparable 
to  the  adult  structure  of  Bouchardia,  Magas,  Magasella,  Tere- 
bratella,  and  Magellania.  The  lower  genera  become  adult 
before  reaching  the  terminal  stages. 

Magellania  Bayle.  Megerlina  Deslongchamps. 

Terebratella  d'Orbigny.       Bouchardia  Davidson. 

Magasella  Dall.  Kraussina  Davidson. 

Magas  Sowerby. 

MEGATHYKIN^E   Dall. 

Loop  composed  of  descending  branches  only,  passing  in  the 
highest  genus  through  stages  correlative  with  G-wynia,  Cis- 
tella,  and  Megathyris.  The  lower  genera  do  not  complete 
the  series. 

Megathyris  d'Orbigny.        Zellania  Moore. 
Cistella  Gray.  Grwynia  King. 

References. 

1.  Agassiz,  L.,  1873.  —  Methods  of  Study  in  Natural  History,  eighth 

edition. 

2.  Crane,  Agnes,  1893.  —  The  Distribution  and  Generic  Evolution  of 

some  Recent  Brachiopoda.     Natural  Science,  vol.  ii. 

3.  Dall,  W.  H.,  1870.  —  A  Revision  of  the  Terebratulida?  and  Lingu- 

lidse  with  remarks  on  and  descriptions  of  some  recent  forms. 
Amer.  Jour.  Conchology,  vol.  vi,  pt.  ii. 


FAMILIES  OF  LOOP-BEARING  BRACHIOPODA      309 

4.  Dall,  W.  H.,  1891.  —  On  some  new  or  interesting  West  American 

Shells  obtained  from  the  dredgings  of  the  U.  S.  Fish  Commission 
Steamer  Albatross  in  1888,  and  from  other  sources.  Proc.  U.  S. 
Nat.  Mus.,  vol.  xiv. 

5.  Davidson,  Thomas,  1876.  —  Monograph  of  the  British  Fossil  Brachi- 

opoda,  Ft.  II.  No.  1.    Pal.  Soc.,  London,  vol.  xxx. 

6.    1886-88.  —  A  monograph  of  Recent  Brachiopoda.    Trans.  Linn. 

Soc.,  London,  vol.  iv. 

7.  Deslongchamps,  E.,  1884.  —  Etudes  critiques  sur  des  Brachiopodes 

nouveaux  ou  peu  connus. 

8.  Fischer,   P.,   and    (Ehlert,   D.-P.,   1892. — Brachiopodes:    Mission 

Scientifique  du  Cap  Horn,  1882-1883.  Bull.  Soc.  Hist.  Nat.  d'Autun, 
vol.  v,  with  5  plates. 

9.   1892.  —  Resultats  des  campagnes  scientifiques  accomplies  sur 

son    yacht  par  Albert   ler,   Prince  Souverain  de  Monaco.      Fas- 
cicule III.     Brachiopodes  de  1'Atlantique  Kord,  2  plates. 

10.   1892.  —  Sur    1'evolution    de    Pappareil    brachial  de    quelques 

Brachiopodes.     Comptes  Rendus,  Nov.,  1892. 

11.  Friele,  H.,  1877.  — [On  the  Development  of  Waldheimia.]    Arch. 

Math.  Nat.,  Bd.  xxiii. 

12.  Gray,  J.   E.,   1848.  —  On  the  Arrangement  of  the  Brachiopoda. 

Ann.  Mag.  Nat.  Hist.  (2),  vpl.  ii. 

13.  Hancock,   A.,   1858. —  On  the  Organization  of  the  Brachiopoda. 

Phil.  Trans.,  vol.  cxlviii. 

14.  King,  W.,  1850.  —  A  Monograph  of  the  Permian  Fossils  of  England. 

Pal.  Soc.,  London. 

15.  Morse,  E.  S.,  1873.  —  On  the  Early  Stages  of  Terebratulina  septen- 

trionalis  (Couthouy).     Mem.  Boston  Soc.  Nat.  Hist.,  vol.  ii. 

16.  Waagen,   W.,   1879-85.  —  Palceontologia  Indica   (13),   voL  i.      Salt 

Range  Fossils. 


4.    DEVELOPMENT   OF  SOME   SILURIAN 
BRACHIOPODA  * 

(PLATES  XV-XXII) 

INTRODUCTION 

THE  fossil  faunas  of  rock  systems  rarely  furnish  mate- 
rial for  tracing  the  individual  development  of  any  of  the 
contained  species.  Much  will  doubtless  be  done  toward 
ascertaining  such  development  when  large  collections  from 
suitable  localities  have  been  studied  with  this  object  in  view, 
and  when  the  number  of  new  species  discovered  and  described 
each  year  approaches  a  minimum.  A  comparatively  full 
and  satisfactory  account  of  the  development  of  the  individual 
organism  in  several  species  of  trilobites  is  given  in  the 
works  of  Barrande,  Waleott,  Ford,  and  Matthew ;  Hyatt, 
Branco,  Mojsisovics,  and  others  have  demonstrated  the  devel- 
opmental characters  of  many  of  the  fossil  cephalopods,  and 
Verworn  has  elicited  similar  facts  from  certain  extinct  species 
of  Ostracoda.  Further  than  this  but  little  has  been  attempted, 
although  the  field  is  a  most  extensive,  important,  and  invit- 
ing one. 

As  a  general  rule,  the  treatment  of  fossil  organisms  has 
rested  mainly  with  geologists  having  more  or  less  of  a  zoolo- 
gical training,  and  the  principal  aim  has  been  to  present  the 
faunal  aspects  of  each  horizon  for  the  purpose  of  chrono- 
logical identification.  This  process  has  frequently  become  so 
involved  with  the  imperfect  description  of  species,  that  the 
systematic  zoologist  or  paleontologist  is  unable  to  make  any 

*  Beecher  and  Clarke.  Mem.  N,  Y.  State  Mus.,  I,  1-95,  pis.  i-viii,  1889. 
The  order  of  the  names  of  the  authors  of  this  paper  is  without  significance. 
The  work  was  equally  divided  and  jointly  reviewed. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     311 

use  of  a  large  proportion  of  the  species  as  a  means  of  study- 
ing their  taxonomic  relations  or  their  structural  affinities 
with  each  other  and  with  recent  forms. 

Each  revision  of  a  group  of  fossil  animals  has  resulted  in 
the  establishment  of  numerous  specific  and  generic  synonyms. 
Many  of  these  are  owing,  of  necessity,  to  the  imperfection  of 
the  material,  and  many  names  which  are  finally  relegated  as 
synonyms  have  been  created  under  a  misconception  of  the 
full  significance  of  age,  sex,  habitat,  and  condition  of  preser- 
vation. Additional  confusion  often  results  from  the  inclu- 
sion, in  a  generic  or  specific  description,  of  characters  which 
pertain  not  alone  to  a  normal  individual,  but  interspersed  with 
certain  normal  adult  features  are  those  belonging  to  various 
stages  of  morphological  development  and  peculiarities  arising 
from  accident,  disease,  and  impoverished  conditions. 

In  the  case  of  rare  species,  or  of  meagre  material  belonging 
to  common  forms,  it  is  to  be  noticed  that  assertions  regarding 
specific  and  generic  characters  are  usually  very  positive; 
while,  with  an  abundance  of  .specimens  representing  many 
stages  of  growth  and  the  extremes  of  individual  variation, 
the  descriptions  are  qualified,  the  latitude  of  genera  and 
species  is  extended,  and  the  points  of  relationship  with  allied 
forms  are  .  multiplied,  thus  binding  a  group  of  organisms  into 
comparative  uniformity,  without  anomalous  differences  such 
as  often  occur  where  the  dividing  lines  are  rigidly  drawn. 

During  the  years  1878-79  the  collection  of  fossils  made 
from  the  Niagara  group  at  Waldron,  Indiana,  for  the  New 
York  State  Museum,  was  studied  and  arranged  by  one  of  the 
writers.  This  is  probably  the  largest  collection  yet  brought 
together  from  that  celebrated  locality,  and  some  conception  of 
its  size  may  be  obtained  from  the  fact  that,  when  received,  it 
weighed  about  seven  tons.  At  the  time  mentioned  all  the 
mature  specimens  were  selected  and  specifically  separated. 
Many  immature  forms  were  also  reserved  and  used  in  arrang- 
ing the  series  prepared  for  exhibition  in  the  State  Museum. 
It  was  designed  to  represent  in  the  arrangement  each  species 
by  a  series  of  specimens  showing  the  gradations  of  size  and 


312  STUDIES  IN  EVOLUTION 

form  from  mature  individuals  down  to  as  young  and  small 
specimens  as  could  be  found.  Abnormal  examples,  also, 
were  reserved  and  grouped  with  them.  It  was  the  intention 
of  the  writers  to  accompany  this  memoir  with  photographic 
illustrations  of  these  series,  representing  each  species  here 
discussed;  but  it  has  not  been  found  wholly  feasible,  and  the 
illustrations  are  largely  restricted  to  the  presentation  of  the 
immature  and  adult  conditions  of  growth,  with  the  exception 
of  the  several  series  which  are  given  on  Plate  XXII. 

The  product  obtained  from  washing  the  slabs  was  pre- 
served and  passed  through  sieves  to  assort  the  material  into 
different  grades  of  fineness.  It  was  found  that  these  wash- 
ings contained  a  great  number  of  partially  developed  shells, 
and  it  is  from  them  that  the  extremely  young  brachiopods 
treated  of  in  the  present  paper  have  been  derived.  The 
writers  have  carefully  examined  all  the  residue  of  these 
washings,  and  have  picked  out  about  fifty  thousand  speci- 
mens, most  of  which  are  less  than  five  millimetres  in  length, 
and  many  have  a  length  of  not  more  than  one  millimetre. 
After  all  the  imperfect  and  badly  preserved  individuals  were 
rejected,  there  still  remained  more  than  fifteen  thousand 
inchoate  individuals. 

The  sediments  at  Waldron  consist  of  fine  calcareous  shales 
weathering  into  clays.  A  stratum  of  Niagara  limestone  over- 
lies the  shales  at  this  locality,  but  none  of  the  fossils  derived 
from  this  limestone  have  been  used  in  the  preparation  of  the 
present  paper,  and  so  far  as  known,  it  has  a  comparatively 
different  fauna  and  does  not  furnish  such  material  as  is  here 
described.  The  calcareous  matter  in  the  shales  consists 
almost  entirely  of  fossils  and  fragments  of  fossils,  principally 
branches  of  corals  and  bryozoa,  segments  of  crinoid  columns, 
and  broken  crinoid  plates.  The  Brachiopoda  are  all  calcare- 
ous, and  the  original  shell  structure  is  more  or  less  preserved, 
depending  upon  the  absence  or  presence  of  pyrite. 

The  occurrence  in  such  great  numbers  of  immature  shells 
in  these  deposits  may  be  explained  by  the  luxuriant  fauna 
which  flourished  in  this  Niagara  basin,  by  the  quiet  seas  of 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     313 

this  region,  and  by  the  rapid  sedimentation  of  the  shales. 
The  richness  of  the  material  is  shown  by  the  great  profusion 
of  specimens  representing  the  sponges,  corals,  crinoids,  Bry- 
ozoa,  brachiopods,  gastropods,  annelids,  and  crustaceans,  com- 
prising altogether  about  one  hundred  and  fifty  species.  The 
lamellibranchs  and  cephalopods  were  also  doubtless  abundant ; 
but  the  conditions  existing  for  the  preservation  of  their 
remains  were  not  favorable,  probably  on  account  of  the  com- 
position of  their  shells,  and  but  sixteen  species  have  been 
noted.  That  the  fauna  was  protected  from  excessive  storms 
and  the  action  of  sea  currents,  is  evinced  by  the  usual  perfec- 
tion of  the  fossils.  Some  of  the  crinoids  are  unbroken  and 
remain  attached  by  their  roots,  retaining  their  arms  in  place ; 
also  large  colonies  of  delicate  branching  corals  and  Bryozoa 
still  preserve  their  unity.  The  specimens  were  rapidly  buried 
in  the  soft  calcareous  mud,  and  show  none  of  the  eroding  or 
disintegrating  action  of  the  water,  such  as  would  have  been 
produced  had  they  lain  for  any  considerable  period  unpro- 
tected on  the  sea-bottom.  It  isftrue  that  many  specimens  are 
incrusted  with  Bryozoa,  annelids,  Cranias,  and  other  fixed  and 
incrusting  forms,  but  the  majority  of  these  seem  to  have 
flourished  during  the  life  of  their  hosts. 

Besides  the  embryonic  Brachiopoda  occurring  in  these 
shales,  there  are  other  classes  represented  by  immature  forms, 
notably  the  Gastropoda  and  Crinoidea.  These,  with  the 
Brachiopoda,  embrace  almost  all  the  young  forms  found.  The 
small  gastropods  are  of  little  interest,  on  account  of  the 
limited  number  of  species,  and  because  they  undergo  no  im- 
portant modification  in  their  subsequent  growth,  and  merely 
represent  the  apical  portion  of  mature  individuals.  Among 
the  crinoids  the  modifications  of  form  and  structure  from  the 
embryo  state  to  maturity  are  more  profound  and  essential, 
although  the  material  is  not  sufficiently  complete  to  furnish 
any  very  important  results. 

It  is  necessary  to  state  that  nearly  all  these  observations  on 
the  development  of  the  Brachiopoda  are  based  upon  the  study 
of  the  material  derived  from  a  single  locality,  and  some 


314  STUDIES  IN  EVOLUTION 

of  the  minor  deductions  may  not  apply,  in  every  case,  to 
the  individuals  of  the  same  species  found  in  other  regions. 
The  writers  have  also  refrained,  except  when  essential  to  the 
proper  exposition  of  a  species,  from  entering  into  details  of 
synonymy  or  generic  controversy  as  to  the  correct  reference 
of  the  species.  This  course  is  considered  advisable,  from  a 
desire  not  to  introduce  any  discussions  alien  to  the  descrip- 
tions of  the  developmental  changes  in  these  organisms.  Aside 
from  this  it  is  believed  that  a  number  of  important  facts  are 
here  added  to  the  knowledge  of  the  Brachiopoda,  and  that 
many  of  them  will  be  found  to  be  of  general  application. 
The  investigation  has  also  resulted  in  elucidating  several 
obscure  and  anomalous  features  of  the  shell  and  of  the  cardi- 
nal area,  which  appear  in  their  proper  place  in  the  description 
of  the  species  and  in  the  general  summary. 

The  following  list  includes  all  the  species  of  Brachiopoda 
which  up  to  this  time  have  been  described  from  the  shales 
at  Waldron,  Indiana,  and  comprises  forty-two  species  and 
varieties,  ascribed  to  twenty-four  genera.  It  also  shows 
whether  material  has  been  obtained  which  furnishes  data  for 
tracing  the  developmental  changes. 

The  majority  of  the  species  which  have  afforded  no  young 
specimens  are  rare  forms  even  in  their  adult  state.  Among 
the  actually  abundant  species  of  which  there  are  no  means 
accessible  of  tracing  the  life-history,  Uncinulus  Stricklandi  is 
a  noticeable  example,  and  it  is  really  the  only  common  species 
which  has  afforded  no  young  shells.  Meristina  Maria,  another 
abundant  form,  furnishes  a  series  which  is  notably  incomplete, 
as  the  youngest  individual  observed,  which  can  with  certainty 
be  referred  to  it,  has  a  length  of  6  mm.  Likewise  the  inarticu- 
late species  have  yielded  almost  no  immature  specimens. 

List  of  the  Brachiopoda  occurring  in  the  Niagara  Shales  at 
Waldron,  Indiana. 

Crania  siluriana  Hall One  embryo. 

Crania  setifera  Hall No  young  shells  obtained. 

Crania  spinigera  Hall "          "          " 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     315 

Lingula  gibbosa  Hall No  young  shells  obtained. 

Pholidops  ovalis  Hall "  "  " 

RUpidomella  hybrida  Sowerby      .     .  Numerous  inchoate  specimens. 

Dalmanella  elegantula  Dalman       .     .  "  "  " 

Orthis  subnodosa  Hall No  young  shells  obtained. 

Bilobites  bilobus  Linnaeus     ....  "  " 

Orthothetes  tennis  Hall "  "  " 

Orthothetes  subplanus  Conrad    •     .     .  Full  series  showing  development. 

Leptcena  rhomboidalis  Wilckens     .     .  "  "  " 

Stropheodonta  profunda  Hall     ...  No  young  shells  obtained. 

StrophonelLa  striata  Hall       ....  Full  series,  showing  development. 

Strophonella  semifasciata  Hall  .     .     .  No  young  shells  obtained. 

Plectambonites  transversalis  Dalman   .  "  " 

Mimulus  waldronensis  Miller  and  Dyer  One  embryo. 

Chonetes  nova-scoticus  Hall  ....  No  young  shells  obtained. 

Chonetes  undulatus  Hall      ....  "  "  " 

Dictyonella  reticulata  Hall   ....  Young  shells  not  rare. 

Clorinda  fornicata,  var.,  Hall  .     .     .  No  young  shells  obtained. 

Anastrophia  internascens  Hall  .     .     .  Young  shells  not  rare. 

Camarotcechia  neglecta  Hall      .     .     .  Young  shells  very  abundant. 

Camarotcechia  acinus  Hall   .... 

Camarotcechia  indianensis  Hall      .     . 

Camarotcechia  Whitii  Hall  .     .     .     / 

Uncinulus  Stricklandi  Sowerby     .     .  Mature  form  abundant ;  no  young 

shells  obtained. 

RJiynchotreta  cuneata  Dalman  .     .     .  Young  shells  common. 

Atrypa  reticularis  Linnaeus        .     .     .  Young  shells  abundant. 

Zygospira  minima  Hall No  young  shells  obtained. 

Atrypina  disparilis  Hall        ....  Young  shells  common. 

Homceospira  evax  Hall Young  shells  very  abundant. 

Homceospira  sobrina  sp.  n Young  shells  not  rare. 

Nucleospira  pisiformis  Hall  *     ... 

Meristina  rectirostris  Hall    ....  Full  series,  showing  development. 

Whitfieldella  nitida  Hall       ....  Numerous  inchoate  specimens. 

Meristina  Maria  Hall Incomplete   series  showing  devel- 
opment. 

Spirifer  eudora  Hall No  young  shells  obtained. 

Spirifer  crispus  Hisinger      ....  Full  series,  showing  development. 

Spirifer  crispus,  var.  simplex,  Hall     .  "  "  ** 

Spirifer  radiatus  Sowerby    ....  ««  "  " 

Reticularia  bicostata,  var.  petila,  Hall.  *'  "  " 

*  The  mature  characters  of  this  species  are  assumed  so  early  that  the 
youngest  forms  observed  show  no  important  differences  from  the  adult.  On 
this  account  no  discussion  of  its  characters  is  given  in  the  ensuing  pages. 


316  STUDIES  IN  EVOLUTION 

The  method  of  illustration  which  has  been  adopted  is  one 
which  seems  most  readily  to  furnish  a  means  for  comparison 
of  characters.  The  embryonic  shells  are  represented  as  en- 
larged, usually  to  the  size  of  an  adult,  and  accompanying  the 
enlargements  are  natural-size  representations  of  the  final 
result  of  normal  growth.  Where  the  mature  forms  have 
been  too  minute  to  show  satisfactorily  the  details  of  struc- 
ture, both  the  developmental  stages  and  full-grown  shell 
have  been  enlarged  to  a  convenient  size.  Thus  the  incipient 
stages  and  mature  specific  form  are  presented  together.  In  the 
delineation  of  special  features,  such  as  the  hinge,  the  writers 
have  sometimes  enlarged  the  earlier  phases  to  a  size  corre- 
sponding with  the  same  structure  in  the  mature  form,  or 
have  increased  all  on  a  uniform  scale,  so  that  both  the  par- 
ticular characters  and  their  comparative  size  are  presented. 

The  enlarged  drawings  have  been  made  by  the  writers, 
principally  from  the  microscope;  the  camera-lucida  was  em- 
ployed to  insure  accuracy  in  outline.  The  illustrations  of 
the  mature  specimens  are  largely  taken  from  the  Twenty- 
eighth  Annual  Report  of  the  New  York  State  Museum  and 
from  the  Eleventh  Report  of  the  State  Geologist  of  Indiana, 
which  may  be  consulted  for  a  more  ample  representation  of 
the  adult  characters  of  the  species  occurring  at  Waldron. 

The  drawings  on  Plates  XV  to  XXI  have  been  reproduced 
on  stone,  in  a  most  satisfactory  manner,  by  Mr.  Philip  Ast, 
and  the  writers  wish  to  express  their  appreciation  of  the  skill 
and  labor  he  has  bestowed  upon  the  work.  The  illustrations 
given  on  Plate  XXII  were  made  from  photographic  repro- 
ductions of  the  actual  series  of  specimens,  and,  although  not 
serviceable  for  purposes  of  detailed  study,  show  distinctly 
the  nature  of  the  material  used  and  the  almost  insensible 
gradations  obtained,  representing  the  life-history  of  these 
species.  The  same  completeness  of  material  is  furnished  by 
the  majority  of  forms  described  in  the  following  pages. 

The  arrangement  of  the  subject-matter  in  the  discussions 
of  the  species  may  not  seem  to  be  in  accordance  with  the 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     317 

usual  method  employed  in  tracing  the  life-history  of  organ- 
isms. In  this  case  fossil  organisms  are  to  be  dealt  with,  and, 
in  order  to  insure  accuracy  of  results,  it  is  necessary  to  begin 
with  the  known  and  established  facts  and  gradually  descend 
to  minute  and  strange  forms,  thereby  connecting  the  extremes 
of  growth.  Under  the  caption  "Developmental  Changes," 
however,  an  endeavor  has  been  made  to  trace  the  history  of 
each  feature  of  the  shell,  from  its  inception  to  maturity. 

DISCUSSIONS  OF  THE  SPECIES. 

Crania  siluriana  Hall,  1863. 

(PLATE  XV,  figures  1,  2.) 

Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat.  Hist., 

p.  148,  pi.  21,  figs.  3-7,  1879. 
Hall.    Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  282,  pi.  21, 

figs.  3-7,  1882. 

But  a  single  embryo  of  this  species  has  been  found  in  all 
the  material  examined.  It  is  of  an  incipient  stage  of  growth, 
measuring  but  1  mm.  in  height,  and  1.5  mm.  across  the 
aperture.  Compared  with  the  mature  form,  the  average  size 
of  which  is  about  9  x  20  mm.,  it  shows  a  relatively  greater 
elevation  and  a  more  regularly  conical  form.  Otherwise  all 
the  few  essential  characters  of  the  adult  shell  are  present  at 
this  early  age. 

Dalmanella  elegantula  Dalman,  1827. 

(PLATE  XV,  figures  3-11.) 

Orthis  elegantula  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat 

Hist.,  p.  150,  pi.  21,  figs.  11-17,  1879. 
Hall.    Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  285,  pi.  21, 

figs.  11-17,  1882. 

Both  species  of  Orthidce  occurring  at  Waldron  (Dalma- 
nella elegantula  and  Rhipidomella  hybridd)  are  very  abun- 
dant. In  the  later  stages  of  growth  the  former  species  is 


318  STUDIES  IN  EVOLUTION 

readily  distinguished  from  the  latter  by  its  flatter  and  shal- 
lower dorsal  valve  and  deeper  ventral  valve,  features  which 
usually  hold  good  for  purposes  of  discrimination;  but  in 
extremely  early  stages  of  growth  the  nearly  equivalve  form 
of  the  shell  makes  the  separation  of  the  species  very  diffi- 
cult, perhaps  even  impossible.  Between  the  dimensions  of 
.5  x  .75  mm.  and  18.5  X  18  mm.  (which  is  a  little  in  excess 
of  average  mature  size)  has  been  found  every  gradation  in 
size  and  development.  The  minute  shell  which  serves  as  a 
starting-point  for  the  series  may  quite  as  well  be  taken  as  the 
incipient  shell  of  R.  hybrida,  as  both  its  valves  have  the  same 
depth,  while  the  cardinal  areas  and  beaks  show  the  same 
character  of  development.  As  there  can  be  no  doubt  of  this 
fact,  it  becomes  impossible  to  determine  whether  a  given 
embryo,  could  it  have  grown  to  maturity,  would  have  devel- 
oped into  R.  hylrida  or  D.  elegantula.  Until  the  embryos 
reach  a  size  of  2  or  2.5  mm.  in  length,  their  specific  value  is 
undeterminable,  and  the  specific  individuality  of  D.  elegantula 
can  be  established  only  with  the  increasing  depth  of  the  ven- 
tral valve  from  this  point  upward  toward  adolescence. 

Unless  these  observations  are  at  fault  (and  they  have  been 
made  with  great  care),  evidence  here  is  very  positive  that 
the  diagnostic  characters  of  species  of  this  group  may  not  be 
assumed  until  the  earlier  stages  of  the  existence  of  the  shell 
have  passed.  Indications  of  similar  character  are  found 
among  the  species  of  Camarotoechia  and  Spirifer.  The  im- 
portance of  the  fact  is  apparent  and  its  significance  will  be 
appreciated. 

Specific  Characters. 

Mature  Form  (Plate  XV,  figures  10-12  a).  —  Outline  sub- 
circular;  hinge-line  short,  about  one-half  the  width  of  the 
shell,  straight. 

Ventral  valve  elevated  along  the  dorsum,  which  is  arched 
and  slopes  more  rapidly  toward  the  lateral  than  toward  the 
anterior  margin ;  greatest  width  below  the  hinge-line,  about 
half-way  down  the  valve.  Beak  full,  arched,  incurved,  and 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA      319 

projecting  over  the  cardinal  area  sufficiently  to  conceal  the 
foramen.  Cardinal  area  broadly  triangular,  low,  incurved; 
foramen  triangular;  deltidial  plates  absent. 

Dorsal  valve  shallow,  nearly  flat,  slightly  rounded  over  the 
umbo,  but  depressed  toward  the  margins.  A  sharply  denned 
sinus  starts  near  the  apex,  but  by  widening  with  the  growth 
of  the  shell,  it  becomes  nearly  obsolete  before  reaching  the 
margins.  Cardinal  line  straight;  cardinal  area  narrow,  elon- 
gate triangular;  beak  inconspicuous.  Foramen  triangular 
and  filled  by  a  tripartite  cardinal  process  which  passes  into, 
without  filling,  the  foramen  of  the  opposite  valve. 

Surface  of  the  shell  closely  covered  by  fine  thread-like 
striae  which  increase  by  intercalation ;  concentric  growth-lines 
rare,  except  near  the  margin,  where  they  appear  as  wrinkles. 

Incipient  Form  (Plate  XV,  figures  3,  3  a).  —  The  initial 
shell  of  the  present  series,  measuring  .5  mm.  in  length  and 
.75  mm.  in  width,  has  valves  of  equal  depth  and  convexity. 
The  length  of  the  hinge-line  nearly  equals  the  greatest  width 
of  the  shell.  The  cardinal  area  fis  high,  and  equally  elevated 
on  each  valve.  Beaks  erect;  foramina  large,  triangular, 
open,  and  marginate.  On  the  ventral  valve  is  a  single  median 
stria,  representing  the  dorsum  of  the  mature  shell,  accom- 
panied by  one  and  indications  of  a  second  on  each  of  the 
lateral  areas,  making  five  striae  on  the  valve.  On  the 
dorsal  valve  a  low  and  wide  median  depression  is  apparent, 
bounded  by  two  central  striaa,  these  being  accompanied  by 
two  accessory  pairs  upon  the  latera,  making  six  strise  in 
all.  It  is  very  probable  that  this  form  represents  nearly  the 
actual  initial  stage  in  the  development  of  the  shell;  and  if 
this  is  the  case,  the  inception  of  the  plications  on  the  sur- 
face, which  become  so  numerous  at  maturity  (from  one 
hundred  to  one  hundred  and  thirty  on  each  valve),  is  syn- 
chronous with  the  formation  of  the  rudimentary  shell,  while 
in  the  pauciplicate  species  here  discussed  they  appear  to  be 
of  secondary  growth. 


320  STUDIES  IN  EVOLUTION 

Developmental  Changes. 

General  Form  and  Outline.  —  In  the  growth  of  the  shell  a 
change  becomes  manifest  in  its  outline  and  relative  propor- 
tions. The  young  stages  have  the  width  greater  than  the 
length,  but  the  more  rapid  axial  growth  of  the  shell  reverses 
these  proportions  at  maturity.  Moreover,  in  the  incipient 
stages,  the  valves,  as  already  noticed,  are  of  nearly  equal 
depth  and  convexity.  In  the  next  stage  the  depth  of  the 
ventral  valve  has  noticeably  increased  over  that  of  the  dorsal, 
and,  as  in  the  latter  valve  the  median  sinus  has  become 
distinctly  developed,  the  difference  in  this  respect  becomes 
emphasized.  The  divergence  of  the  valves  in  convexity  be- 
comes increased  until  maturity,  and  this  growth  is  accom- 
panied in  the  ventral  valve  by  a  correspondingly  increasing 
incurvature  of  the  beak. 

Beaks.  —  111  the  incipient  shell  the  beaks  are  erect  and 
distant,  but  not  prominent.  By  the  development  of  the 
broad  sinus  on  the  dorsal  valve,  the  beak  of  this  valve  be- 
comes relatively  less  prominent  and  apparently  more  closely 
appressed  to  the  cardinal  line.  On  the  opposite  valve  every 
increase  in  convexity  is  accompanied  by  a  corresponding 
increase  in  the  incurvature  of  the  beak;  and  as  the  shell 
approaches  maturity,  the  incurvature  becomes  so  great  that 
it  has  been  necessary,  in  the  drawings  which  are  here  given 
showing  the  features  of  the  cardinal  area,  to  represent  the 
beak  as  broken  away. 

Foramen.  —  The  earliest  stages  of  growth  show  a  remark- 
able feature  in  the  triangular,  marginate,  sub-equal  fissures 
on  the  valves.  This  character  may  prove  of  a  high  taxo- 
nomic  value.  The  foramen  upon  the  ventral  valve  is,  in 
every  stage  of  development,  open  and  free  for  the  pro- 
trusion of  the  pedicle.  Deltidial  plates  are  absent  in  every 
stage  of  growth.  In  a  secondary  stage  a  cardinal  process 
begins  to  form  in  the  apex  of  the  dorsal  foramen,  soon  widen- 
ing and  becoming  tripartite.  As  age  increases,  this  process 
is  projected  into  the  ventral  foramen,  never  quite  filling  it, 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     321 

always  leaving  room  for  the  protrusion  of  the  pedicle.  In 
immature  conditions  the  cardinal  process  is  attached  to  the 
shell  only  at  the  apex  of  the  foramen,  but  with  maturity  it 
comes  in  contact  with  the  sides  of  the  foramen,  and  at  this 
stage  entirely  fills  the  dorsal  aperture.  With  the  increasing 
incurvature  of  the  ventral  beak  and  cardinal  area,  the  aper- 
tures of  the  two  valves  change  their  mutual  angle,  constantly 
lessening  it  as  growth  advances. 

Plications.  —  As  noticed  above,  the  earliest  stages  of  growth 
observed  show  the  strise  to  be  already  developed  on  the  shell, 
five  on  the  ventral  and  six  on  the  dorsal  valve.  These  plica- 
tions are  rapidly  multiplied  by  interstitial  addition,  and  at 
maturity  number  from  one  hundred  to  one  hundred  and 
thirty  on  each  valve. 


Rhipidomella  hybrida  Sowerby,  1839. 

(PLATE  XV,  figures  13-18.) 

Orthis  hybrida  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  149,  pi.  21,  figs.  18-25,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  285,  pi. 

21,  figs.  18-25,  1882. 

Rhipidomella  hybrida  passes  through  primary  develop- 
mental stages  which  are  essentially  identical  with  those 
already  described  for  Dalmanella  elegantula.  Sufficient  has 
been  said  in  that  connection  in  regard  to  the  similarity  and 
probable  identity  of  the  earlier  embryonic  stages  of  the  shell 
of  both  species,  the  origin  of  the  entire  specific  difference 
which  is  so  apparent  in  the  later  and  mature  periods  of  devel- 
opment lying  in  the  unequal  growth  of  the  valves  in  con- 
vexity. This  increase  is  relatively  greater  in  the  dorsal  valve 
of  R.  hybrida  than  in  that  of  D.  elegantula,  and  less  in  the 
ventral  valve  of  the  former  than  in  that  of  the  latter  species. 
Thus  R.  hybrida  is  a  more  discoid,  lenticular  shell,  showing 
but  slignt  evidence  of  a  median  fold  and  sinus  and  carrying 

21 


322  STUDIES  IN  EVOLUTION 

on  its  surface  at  maturity  just  about  as  many  plications  or 
strise  as  its  associate. 

There  is  an  obese  variation  from  the  normal  form  of 
R.  liybrida,  which  was  noticed  by  Professor  Hall  (loc.  cit.), 
and  this  appears  early  in  the  development  of  the  species, 
with  a  size  of  3.5  mm.  in  length  and  4.5  mm.  in  width,  and 
reaches  a  maximum  growth  with  dimensions  of  14  x  13  mm. 
This  variation  is  due  to  internal  thickening  and  increase 
in  convexity,  and  is  accompanied  by  abundant  concentric 
growth-lines  which  are  as  rare  in  the  normal  form  as  in 
D.  elegantula.  A  representative  series  of  this  species 
affords  variations  between  the  following  limits  of  size :  .5  mm. 
in  length  x  .75  mm.  in  width  (minimum),  and  17  mm.  in 
length  x  20  mm.  in  width  (maximum). 

Leptcena  rhomboidalis  Wilckens,  1769. 

(PLATE  XVI,  figures  1-13.) 

StropTiomena  rhomboidalis  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State 

Mus.  Nat.  Hist.,  p.  151,  pi.  22,  figs.  4-10,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  288.  pi.  22, 

figs.  4-10,  1882. 

This  well-known  species,  although  extremely  abundant  in 
the  mature  state,  is  correspondingly  rare  in  its  undeveloped 
condition.  The  young  specimens  which  have  been  found  are 
nearly  all  more  or  less  broken,  and  it  is  evident  that  while 
young  the  shell  was  thin  and  delicate,  consequently  few  of 
their  remains  have  been  preserved.  The  series  which  has 
been  selected  is,  however,  very  complete  in  its  representation 
of  the  distinct  phases  of  growth  through  which  the  individ- 
uals pass  in  their  development  from  youth  to  maturity. 
The  initial  form,  without  radiating  striae;  the  second 
phase,  a  shell  radiatingly  striate,  without  undulations;  the 
third  state,  striated  and  concentrically  undulated,  but  with- 
out the  angular  geniculation  of  the  valves  in  front;  and  the 
last  phase,  with  the  full  form  and  characters  of  maturity, 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     323 

offer  a  series  of   changes,    not   often   traceable   in   Silurian 
brachiopods. 

The  development  of  the  characters  of  the  hinge-area  is  also 
very  satisfactorily  demonstrated,  and  affords  some  interesting 
points  of  comparison  with  certain  forms  of  Orthothetes  and 
Strophonella.  These  features  are  noticed  at  the  end  of  the 
description  of  the  species  Strophonella  striata. 

Specific  Characters. 

Mature  Form  (Plate  XVI,  figures  4,  4  a,  10,  13).  —Shell 
semi -elliptical  or  semi-circular  in  outline. 

Dorsal  valve  flat  or  slightly  concave  in  the  upper  part, 
with  the  marginal  portions  abruptly  curved  upward  in  front ; 
beak  small,  carrying  on  its  inner  side  a  large,  prominent,  tri- 
angular callosity,  grooved  along  its  summit  and  nearly  filling 
the  area  of  the  opposite  valve. 

Ventral  valve  usually  convex  in  the  upper  part,  becoming 
flat  or  concave  below,  and  with  the  marginal  portion  pro- 
duced and  abruptly  bent  dowrtward,  geniculating  with  the 
dorsal  valve;  beak  small,  usually  perforated  with  a  small 
circular  foramen;  hinge-line  often  50  mm.  in  length,  equal- 
ing or  greater  than  the  width  of  the  shell  below;  cardinal 
extremities  twisted  and  often  much  extended ;  cardinal  area 
narrow,  edges  parallel,  formed  by  both  valves ;  deltidial  area 
broadly  triangular,  occupied  by  the  grooved  callosity  under 
the  dorsal  beak. 

Surface  marked  by  regular,  rounded,  radiating  striae. 
From  the  beaks  to  the  curtain,  or  geniculated  portion,  the 
shell  is  ornamented  with  regular,  strong,  concentric  undula- 
tions or  corrugations. 

This  species  varies  greatly  in  size  and  form,  in  the  different 
horizons  and  localities  where  it  is  found.  In  many  places 
the  mature  shells  are  about  half  the  size  of  the  specimens 
from  Waldron. 

incipient  Form  (Plate  XVI,  figures  1, 1  a,  11).  —  The  small- 
est entire  specimen  yet  detected  has  a  length  of  1.25  mm. 
The  outline  is  semi-oval,  with  the  greatest  width  near  the 


324  STUDIES  IN  EVOLUTION 

middle,  and  about  one -fourth  greater  than  the  length.  Dorsal 
valve  convex  in  the  upper  part,  becoming  concave  toward  the 
front.  The  hinge-area  of  this  valve  is  very  narrow  and 
linear,  and  carries  beneath  the  beak  a  small  grooved  callosity. 
Ventral  valve  convex,  sloping  in  all  directions  from  near 
the  foramen,  around  which  the  surface  is  slightly  depressed. 
There  is  also  a  depression  extending  along  the  middle  of  the 
valve  to  the  anterior  margin.  The  place  of  the  beak  is  occu- 
pied by  an  exsert,  conical  pedicle-tube,  which  partly  pro- 
trudes beyond  the  cardinal  margin  of  the  valve  and  extends 
down  to,  and  embraces  the  dorsal  callosity.  Cardinal  area 
of  the  ventral  valve  comparatively  broad,  narrowing  rapidly 
from  the  pedicle-tube  to  the  extremities.  Surface  smooth, 
except  along  a  narrow  zone  around  the  margin,  which  shows 
incipient  radiating  striae. 

Developmental  Changes. 

The  form  of  this  species  being  somewhat  complex,  the 
development  of  the  shell  may  be  conveniently  sub-divided 
into  four  stages,  briefly  characterized  as  follows :  — 

1st  Stage.    Length  of  shell  .4-1  mm. ;  surface  smooth. 

2d  Stage.    Length  1-2  mm. ;  shell  radiatingly  striated,  with- 
out undulations. 

3d  Stage.    Length  2-20  mm. ;  shell  radiatingly  striated,  and 

concentrically  undulated. 

4th  Stage.  Length  20-30  mm. ;  entire  shell  radiatingly  stri- 
ated, concentrically  undulated  in  the  upper 
part,  abruptly  produced  and  geniculated  in 
front. 

The  changes  taking  place  in  the  form  and  character  of  the 
shell  from  one  stage  to  another  can  be  best  shown  and  used 
for  comparison  in  the  following  tabulation,  where  the  condi- 
tions incident  to  each  stage  of  growth  in  the  various  parts  of 
the  shell  are  briefly  described :  — 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA      325 


Development  of  Leptcena  rhomboidalis. 


1st  Stage. 

Initial. 

2d  Stage. 

Infantile. 

3d  Stage. 
Adolescent. 

4th  Stage. 
Mature. 

Size 

.4  mm.  —  1  mm. 

1mm.  —  2  mm. 

2  mm.  —  20mm. 

20     mm.  —  30 

in    length,   .4 

in  length,  1.5 

in  length,  2.5 

mm.inlength, 

mm.  —  1.5mm. 

mm.  —  2.5mm. 

mm.  —  40  mm. 

40    mm.  —  50 

in  width. 

in  width. 

in  width. 

mm.  in  width. 

Transversely 

Transversely 

Longitudinally 

Longitudinally 

semi-oval;  car- 

semi-ellipti- 

semi-ellipti- 

semi-ellipti- 

dinal extrem- 

cal; cardinal 

cal;  cardinal 

cal  ;  cardinal 

ities  obtusely 

extremities 

extremities 

extremities 

angular. 

angular. 

angular,    be- 

acutely angu- 

coming   pro- 

lar,   extended 

duced,       not 

and     twisted. 

twisted. 

Contour 

Convex. 

Depressed  con- 

Very   slightlv 

Geniculate, 

vex. 

convex. 

making    the 

shell    highly 

arched     lon- 

1 

gitudinally. 

Dorsal  valve  .  . 

Convex,    con- 

Concave, except 

Concave,     ex- 

Flat or  concave 

cave    on  the 

on  the  umbo. 

cept    on    the 

on  the  body  of 

margin;  umbo 

umbo. 

the  shell  ;  ab- 

prominent. 

r  u  p  tl  y    pro- 
duced   and 

curved  up- 

ward    around 

the  margins. 

Ventral  valve  . 

Convex. 

Convex,  semi- 

Convex  in  the 

Convex  in  the 

conical    with 
the    beak   at 

upper     part, 
flat  or  concave 

upper    part, 
flat  or  concave 

the  apex. 

on  the  margin. 

in  the  middle, 

and    abruptly 

bent  down- 

ward   below. 

Surface   

Smooth. 

Radiatin  gly 
striate. 

Radiatingly 
striate      and 

Entire   surface 
radiatingly 

concentrically 
undulate. 

striate,concen- 
trically  undu- 

late in  the  up- 

per part  only. 

326  STUDIES  IN  EVOLUTION 

Development  of  Leptcena  rhomboidalis.  —  Continued. 


1st  Stage. 
Initial. 

2d  Stage. 
Infantile. 

3d  Stage. 
Adolescent. 

4th  Stage. 
Mature. 

Cardinal  area  . 
Pedicle-tube  .  . 

Foramen  
Dorsal  callosity. 

Ventral  high  ; 
dorsal  very 
slender. 

Ventral  high; 
dorsal  very 
slender. 

Ventral  nar- 
row ;    dorsal 
narrow. 

Both  narrow, 
sub-equal. 

Exsert,  full 
height  of  the 
area. 

Not  exsert,  full 
height  of  the 
area. 

Nearly  full 
height  of  the 
area. 

Obsolescent  or 
obsolete. 

Present,  circu- 
lar, elevated. 

Present,  circu- 
lar. 

Present,  circu- 
lar. 

Usually  pres- 
ent. 

Small,  grooved. 

Small,  grooved. 

Larger,  grooved. 

Very  large 
and  deeply 
grooved. 

Among  the  mature  shells  the  greatest  variation  is  to  be 
found  in  the  development  of  the  anterior  curtain,  or  genicu- 
late  and  sloping  marginal  area  of  the  valves.  In  some  speci- 
mens this  is  so  excessively  developed  that  the  posterior  or 
concentrically  undulated  portion  of  the  ventral  valve  is  at 
right  angles  to  the  plane  of  the  margin.  Also,  in  many 
specimens  the  curtain  is  obscurely  plicate,  and  the  radiating 
striae  are  often  irregular  and  sometimes  fasciculate,  while  on 
the  upper  part  of  the  valves  these  striae  are  very  uniform 
in  their  arrangement.  No  specimens  have  been  noticed 
which  are  so  strongly  quadriplicate  as  those  illustrated  by 
Mr.  Davidson,  on  Plate  XXXIX  of  the  "British  Silurian 
Brachiopoda. " 

Senile  specimens  usually  have  the  valves  very  much  thick- 
ened from  internal  growth,  and  the  margins  show  strong 
varices.  It  is  noticeable  that  nearly  all  the  old  shells  are 
covered  with  a  growth  of  Cranias,  Bryozoa,  Favosites,  etc., 
and  it  is  very  difficult  to  free  the  shell  from  this  overgrowth. 
In  consequence  of  this,  many  of  the  shells  are  scarcely  recog- 
nizable, and  resemble  agglomerations  of  Bryozoa  and  corals. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     327 

The  only  other  species  of  Brachiopoda  at  this  locality  com- 
monly thus  overgrown  and  involved  is  Atrypa  reticularis. 

Leptcena  rJwmboidalis  is  cosmopolitan  and  has  been  dis- 
cussed by  many  authors,  who  have  shown  its  great  variation 
and  wide  distribution.  So  far  as  known,  the  youngest  speci- 
men heretofore  figured  is  one  represented  by  Mr.  Davidson.* 
This  is  an  individual  belonging  to  the  third  stage  of  develop- 
ment, having  a  length  of  nearly  6  mm.  and  a  distinct  cir- 
cular perforation  of  the  beak. 

Orthothetes  subplanus  Conrad,  1842. 

(PLATE  XVI,  figures  14-20.) 

Streptorhynchus  subplanum  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State 

Mus.  Nat.  Hist.,  p.  151,  pi.  21,  figs.  26-33,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  288,  pi.  21, 

figs.  26-33,  1882. 

The  series  selected  to  represent  the  development  of  this 
species  comprises  fourteen  specimens  ranging  from  1.5  mm. 
to  26.5  mm.  in  length.  The  external  features  of  form  and 
surface  ornament  are  remarkably  constant  from  the  young  to 
the  mature  shells.  There  is,  however,  a  slight  progressive 
modification  in  the  relative  convexity  of  the  valves.  The 
dorsal  valve  of  young  and  half-grown  individuals  is  nearly 
flat,  while  the  ventral  is  moderately  convex.  In  old  speci- 
mens both  valves  are  convex,  with  the  dorsal  somewhat  more 
so  than  the  ventral.  The  most  marked  changes  due  to 
advancing  growth  are  those  which  take  place  in  the  hinge. 
Some  mention  of  these  is  made  under  the  description  of 
Strophonella  striata,  where  it  is  stated  that  the  pedicle-tube 
retains  its  embryonic  form  and  size  nearly  up  to  maturity, 
after  which  it  is  obscured  by  the  internal  thickening  of  the 
shell;  also,  that  the  callosity  under  the  beak  of  the  dorsal 

*  British  Fossil  Brachiopoda,  III,  Devonian  and  Silurian,  283,  284,  pi. 
xxx,  fig.  6.  The  same.  —  General  Summary  to  the  British  Fossil  Brachiopoda, 
289. 


328  STUDIES  IN  EVOLUTION 

valve  uniformly  increases  in  size  from  the  youngest  forms  to 
full-grown  specimens. 

Specific  Characters. 

Mature  Form  (Plate  XVI,  figures  15,  15  a,  17,  20).  — Shell 
semi-circular  or  semi-elliptical,  depressed  convex ;  hinge-line 
longer  than  the  width  of  the  shell;  cardinal  angles  flat  and 
extended. 

Dorsal  valve  moderately  and  uniformly  convex  except  at 
the  cardinal  angles;  umbo  not  defined;  beak  small. 

Ventral  valve  convex  on  the  umbo,  less  convex  below,  and 
in  many  specimens  the  marginal  portion  is  flat  or  slightly 
concave;  beak  small,  somewhat  arched.  Hinge-area  nearly 
equal  in  both  valves,  usually  appearing  as  a  deep  angular 
groove  along  the  cardinal  margin.  Under  the  beak  of  the 
dorsal  valve  is  a  large  triangular  callosity,  grooved  on  the 
inside,  and  nearly  filling  the  fissure  of  the  opposite  valve. 
Deltidium  of  the  ventral  valve  broadly  triangular,  extend- 
ing to  just  below  the  beak,  and  margined  on  each  side  by 
two  narrow  areas  in  the  form  of  scalene  triangles,  which 
may  represent  the  deltidial  plates  of  other  genera.  Beak 
imperf  orate. 

Surface  marked  by  from  fifty  to  one  hundred  (according 
to  the  size  of  the  shell)  regular,  rounded  striae,  with  equal 
interspaces,  increasing  in  number  by  interstitial  additions. 
The  entire  shell  is  also  ornamented  with  very  fine,  regular, 
sharp,  concentric  strise.  A  large  specimen  has  a  length  of 
26  mm.,  and  the  width,  measured  along  the  hinge-line,  is 
about  38  mm. 

Incipient  Form  (Plate  XVI,  figures  14,  14  a).  —  The  small- 
est specimen  measures  1.5  mm.  in  length  by  2.3  mm.  in 
width  along  the  hinge-line.  The  outline  is  semi-elliptical, 
with  the  cardinal  angles  slightly  extended.  Dorsal  valve 
concave  in  the  upper  part,  and  slightly  convex  below.  Ven- 
tral valve  convex;  beak  prominent,  projecting  beyond  the 
hinge-line. 

The  hinge  characters  are  not  well  preserved  in  this  indi- 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     329 

vidual.  The  first  specimen  in  the  ascending  series  which 
shows  the  hinge  distinctly  has  a  length  of  2.25  mm.,  and  will 
be  described  in  the  development  of  this  part. 

The  surface  of  the  incipient  shell  is  marked  by  seventeen 
alternating,  narrow,  elevated  radiating  lines,  with  wider 
interspaces,  and  also  shows  several  lines  of  growth  near  the 
margin. 

Developmental  Variations. 

No  marked  changes  occur  in  the  general  form  of  the  shell 
other  than  the  gradual  increase  in  the  convexity  of  the  dorsal 
valve  and  in  the  extension  of  the  cardinal  angles.  The 
dorsal  valve  is  usually  quite  flat  in  specimens  having  a 
length  of  10  mm.  or  less.  The  radiating  lines  increase  in 
number  by  interstitial  additions,  from  the  youngest  form  to 
maturity,  and  the  fine  concentric  striae  appear  on  all  the 
specimens,  including  the  initial  individual  in  the  series, 
where  they  are  developed  around  the  margins  of  the  valves. 

The  earliest  phase  of  the  hiiage  yet  noticed  is  found  in  a 
specimen  having  a  length  of  2.25  mm.  The  dorsal  valve 
shows  a  foramen  in  the  cardinal  area  under  the  beak,  mar- 
gined by  a  slight  thickening  of  the  shell.  The  ventral  valve 
preserves  a  small  perforate  pedicle-tube  at  the  apex,  extend- 
ing about  two-thirds  of  the  distance  down  to  the  hinge,  below 
which  is  a  triangular  deltidial  opening  of  the  same  width  as 
the  dorsal  foramen. 

A  specimen  4  mm.  in  length  (Plate  XVI,  figure  19)  shows 
a  more  advanced  development  of  the  same  parts.  The  dorsal 
callosity  has  nearly  filled  the  sinus  under  the  beak  and  has 
a  narrow  groove  in  the  centre.  The  fissure  of  the  ventral 
valve  has  increased  considerably  in  size  and  relative  height, 
showing  narrow  marginal  plates  or  defined  areas.  The 
pedicle-tube  is  still  perforate,  but  has  not  increased  in  size 
beyond  the  initial  stage. 

From  this  point  to  maturity  the  hinge  increases  in  width, 
the  dorsal  callosity  grows  rapidly  and  nearly  fills  the  fissure 
of  the  opposite  valve.  The  pedicle-tube  is  obscured,  and  the 


330  STUDIES  IN  EVOLUTION 

perforation  obsolete.  The  marginal  plates,  or  lateral  areas, 
are  clearly  defined,  and  have  the  form  of  narrow  scalene 
triangles. 

No  important  variations  have  been  noticed  among  the 
mature  specimens.  Occasionally  an  individual  diverges  from 
the  normal  form  by  having  mucronate  cardinal  angles,  or  a 
senile  specimen  shows  strong  imbricating  varices  of  growth ; 
but,  as  a  whole,  the  form  and  surface  ornaments  in  this 
species  are  very  uniform. 

Strophonella  striata  Hall,  1843. 

(PLATE  XVII,  figures  1-8.) 

Strophodonta  striata  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 
Nat.  Hist.,  p.  152,  pi.  23,  figs.  1-6,  1879. 

Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  290,  pi.  23, 

figs.  1-6,  1882. 

The  present  form  is  one  of  the  most  delicate  and  fragile 
species  of  Brachiopoda  at  Waldron.  Individuals  are  not  of 
rare  occurrence,  but  the  majority  of  them  are  more  or  less 
broken.  The  upper  portion  of  the  shell,  or  that  along  the 
hinge,  being  thicker  and  stronger  than  the  remainder,  is  more 
often  preserved,  and  the  series  is  only  complete  in  the  repre- 
sentation of  this  portion,  although  there  are  several  small 
specimens  which  are  sufficiently  entire  to  show  the  early 
form  of  the  shell. 

As  in  the  other  species  which  in  their  mature  proportions 
depart  from  the  type  of  structure  in  the  group,  the  incipient 
shell  is  found  to  revert  to  the  primitive  form.  The  full-grown 
examples  of  this  species  are  concavo-convex,  the  concave 
valve  being  the  ventral ;  while  in  the  young  the  ventral  valve 
is  the  more  convex.  This  change  in  the  relative  convexity 
of  the  valve  does  not  begin  until  the  individuals  are  about 
half  grown,  and  is  produced  by  the  gradual  deflection  of  the 
margin  with  the  increase  in  the  size  of  the  shell. 

The  development  of  the  features  of  the  hinge  is  very  char- 
acteristic, and,  as  in  the  other  strophomenoid  forms,  is  of 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     331 

primary  interest.  Both  the  dorsal  callosity  and  pedicle -tube 
continue  to  increase  in  size  with  the  growth  of  the  shell, 
from  the  incipient  form  to  maturity. 

Specific  Characters. 

Mature  Form  (Plate  XVII,  figures  2,  2a,  8).  —  Shell  semi- 
elliptical,  wider  than  long,  the  greatest  length  being  along 
the  hinge.  The  body  cavity  is  very  shallow,  and  the  shell 
has  a  concavo-convex  form. 

Dorsal  valve  flat  in  the  upper  part,  moderately  convex  in 
front.  Ventral  valve  slightly  convex  on  the  umbo,  and  con- 
cave over  the  remainder  of  the  valve.  Hinge-area  formed  by 
both  valves.  Ventral  area  the  wider,  carrying  in  the  centre 
a  small  conical  pedicle -sheath  which  is  usually  minutely 
perforate  at  the  apex.  Dorsal  area  linear,  with  a  callosity  in 
the  middle,  under  the  pedicle -tube  of  the  opposite  valve. 

Test  thin,  surface  ornamented  by  about  fifty  alternating 
radii,  with  three  or  four  fine  filiform  striae  in  each  interspace ; 
also  crossed  by  fine  irregular  stride  of  growth. 

Two  specimens  measure,  respectively,  19.5  mm.  and  14  mm. 
in  length,  and  23  mm.  and  16  mm.  in  width,  at  the  hinge. 

Incipient  Shell  (Plate  XVII,  figures  1,  la,  3).  —  The  form 
is  nearly  plano-convex.  Dorsal  valve  convex  on  the  umbo, 
flat  below.  Ventral  valve  moderately  convex,  with  a  promi- 
nent pointed  beak.  Hinge  narrow,  with  a  small  cylindrical 
perforated  pedicle-tube  in  the  centre  of  the  ventral  area,  and 
a  small  callosity  in  the  dorsal  area.  In  the  smallest  speci- 
men observed  the  surface  is  marked  by  eleven  radii  on  the 
ventral  valve,  but  is  otherwise  apparently  smooth.  Length 
2,25  mm.;  width  in  the  centre  3  mm. 

Developmental  Changes. 

On  account  of  the  imperfection  of  the  material,  it  is  impos- 
sible to  trace  any  minor  changes  in  the  outline  of  the  valves, 
and  the  specimens  indicate  that  no  considerable  transforma- 


332  STUDIES  IN  EVOLUTION 

tion  took  place.  The  modifications  in  the  convexity  of  the 
valves  is  of  more  importance  in  this  species,  and  can  be  read- 
ily observed.  In  the  young  individuals,  up  to  about  one- 
third  full  size,  the  ventral  valve  is  slightly  convex  and  the 
dorsal  valve  nearly  flat.  Further  growth  of  the  shell  changes 
these  relations,  by  the  gradual  deflection  of  the  margin,  until 
the  general  form  of  the  ventral  valve  is  concave  and  the  dorsal 
valve  is  convex. 

The  radii  appear  very  early  in  the  growth  of  the  shell,  the 
smallest  individual  having  eleven  on  the  ventral  valve,  the 
majority  of  which  extend  to  the  umbo.  They  probably  first 
appeared  in  pairs,  and  are  found  to  increase  in  number  after- 
ward by  simple  intercalation. 

The  hinge-area  is  developed  upon  both  valves  in  all  stages 
of  growth,  although  in  the  early  stages  the  cardinal  area  of 
the  dorsal  valve  is  very  narrow,  but  gradually  increases,  until 
at  maturity  it  is  nearly  equal  to  the  area  of  the  ventral  valve. 

The  pedicle-tube  is  at  first  cylindrical  and  short.  Advanc- 
ing in  the  series,  it  is  found  to  become  conical  from  growth 
and  from  the  widening  of  the  fissure,  until  in  full-grown 
specimens  it  is  wider  than  high.  A  careful  examination 
reveals  the  perforation  in  all  stages  of  the  development  of 
the  shell.  It  is,  however,  very  minute,  and  it  is  not  prob- 
able that  the  extremely  small  peduncle  could  have  performed 
its  full  function.  Indeed,  it  may  be  surmised  that  in  none 
of  the  three  strophomenoid  species  here  described  was  the 
fleshy  arm  sufficiently  strong  in  mature  individuals  to  serve 
as  a  secure  support  to  the  shell.  In  the  embryonic  forms  it 
was  a  more  important  organ. 

The  hinge  of  the  young  shell  illustrated  in  figure  4,  Plate 
XVII,  shows  an  excessively  elongate,  cylindrical  pedicle- 
tube,  of  which  more  than  one-half  the  length  is  projected 
above  the  beak.  It  must  be  considered  as  a  supra-calcifica- 
tion  about  the  peduncle,  and  apparently  indicates  a  more 
complete  functional  extension. 

The  grooved  dorsal  callosity  appears  in  the  beginning  of 
the  series,  and  gradually  increases  in  size,  and  detrudes  so 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     333 

that  the  groove  shows  on  the  exterior,  but  just  before  matur- 
ity it  is  either  filled  or  introverted  into  the  deltidial  cavity. 

The  features  of  the  hinge,  fissure,  and  callosity,  in  the 
Strophomenidse,  and  their  embryological  development,  seem 
to  be  peculiar  to  the  group.  They  are  of  special  interest, 
both  on  this  account  and  also  because  the  family  has  no 
living  congeners.  Although  the  separate  characters  have 
been  presented  in  detail  in  each  of  the  preceding  descrip- 
tions, a  brief  review  of  the  hinge  characters  is  here  given, 
showing  more  clearly  their  intimate  relationships. 

In  the  three  species,  Leptcena  rhomboidalis,  Strophonella 
striata,  and  Orthothetes  subplanus,  the  initial  form  of  the  hinge 
is  the  same.  Each  shows  a  slender  callosity  under  the  beak 
of  the  dorsal  valve,  and  a  perforate  pedicle-sheath  in  the 
ventral  valve,  which  does  not  entirely  close  the  deltidial 
opening.  From  this  initial  stage  development  proceeds  in 
a  different  manner  for  each  of  the  three  species.  Leptcena 
rhomboidalis  and  Strophonella  striata  develop  in  a  parallel 
series  until  the  individuals  are  about  one-third  grown  in  the 
first  species,  and  two-thirds  full  size  in  the  second.  That  is, 
the  dorsal  callosity  and  pedicle-sheath  each  increase  uni- 
formly in  size  up  to  these  periods.  Beyond  this  the  diver- 
gence is  rapid  and  marked.  In  Leptcena  rhomboidalis  the 
grooved  callosity  increases  in  size  so  as  to  nearly  fill  the 
broad  fissure  in  the  ventral  valve,  while  the  pedicle-sheath 
ceases  growth,  is  atrophied  and  lost,  although  in  many  cases 
the  perforation  persists.  Strophonella  striata  continues  its 
hinge  development  without  change,  except  that  at  full 
maturity  the  groove  on  the  callosity  becomes  introverted 
into  the  pedicle-sheath. 

The  third  mode  of  development  is  exhibited  by  Orthothetes 
subplanus,  in  which  the  pedicle-sheath  does  not  increase 
beyond  its  initial  size,  while  the  dorsal  callosity  develops  up 
to  the  maturity  of  the  shell,  and,  as  in  Strophonella  striata, 
the  groove  is  on  the  inner  side. 

The  function  of  this  groove  in  the  callosity  of  the  dorsal 


334  STUDIES  IN  EVOLUTION 

valve  in  the  strophomenoids  has  not  been  satisfactorily  deter- 
mined, its  existence  having  sometimes  been  considered  as 
evidence  of  the  perforation  of  this  valve.*  In  all  young 
shells  it  is  evident  that  the  passage  of  the  pedicle  is  not 
through  this  groove  in  the  dorsal  callosity,  but  through  the 
apex  of  the  ventral  valve  by  means  of  the  channel  which  has 
been  here  termed  the  pedicle-tube  or  sheath.  In  growth- 
stages  where  there  can  be  no  question  of  the  functional  activ- 
ity of  this  sheath,  the  dorsal  callosity  is  already  grooved  or 
sinuate.  It  might  be  surmised  that  the  purpose  of  the 
groove  was  to  avoid  compressing  the  pedicle  when  the  valves 
were  open,  and  this  it  may  have  been  to  some  extent;  but 
the  evidence  furnished  by  both  recent  and  fossil  species  indi- 
cates that  the  valves  of  the  articulate  brachiopods  could  be 
opened  only  to  a  very  slight  degree.  The  groove  persists  in 
species  after  the  true  pedicle  perforation  in  the  ventral  valve 
is  closed  and  functionally  useless.  Its  origin  appears  to  be 
due  to  the  organic  deposition  about  the  bases  of  the  two 
interior  cardinal  processes,  the  interstitial  area  of  slower 
deposition  being  represented  by  a  fissure,  groove,  or  sinus. 

Mimulus  waldronensis  Miller  and  Dyer,  1878. 

(PLATE  XVII,  figures  9,  10.) 

Spirifera  f  waldronensis  Miller  and  Dyer.     Contributions  to  Palaeontology, 

Jour.  Gin.  Soc.  Nat.  Hist.,  April,  1878. 

Triplesia  putillus  Hall.     Trans.  Alb.  Inst.,  vol.  x,  abstract,  p.  16,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  298,  pi.  27, 

figs.  19-22,  1882. 

This  species  is  among  the  rarest  of  the  Waldron  Brachi- 
opoda,  and  it  is  impossible  to  present  a  series  representing  the 
variety  and  progress  of  development,  as  in  some  of  the  more 
common  forms.  There  were  but  two  specimens,  both  adults, 
discovered  in  the  State  Collection  at  the  time  of  the  publica- 
tion of  the  "  Descriptions  of  New  Species  of  Fossils  from  the 

*  Eleventh  Rept.  State  Geologist  Indiana,  288,  289,  1882. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     335 

Niagara  Formation  at  Waldron,  Indiana."*  Fortunately, 
there  has  been  more  recently  detected  a  young  individual  of 
about  one-fourth  the  normal  adult  size,  which  offers  some 
interesting  details  in  its  form  and  characters. 


The  asymmetry  of  the  shell  is  manifest  even  at  this  early 
stage  of  growth  (although  the  median  fold  is  not  developed), 
and  is  evinced  by  the  position  of  the  beak  of  the  ventral 
valve  and  by  the  contour  of  the  margins.  It  is  probable 
that  in  a  still  earlier  phase  of  growth  the  two  valves  are 
symmetrical,  or  nearly  so. 

In  the  young  individual  under  consideration  (Plate  XVII, 
figures  9,  9  a,  96),  the  outline  is  nearly  circular.  The  beak 
of  the  ventral  valve  is  very  much  elevated,  projects  beyond 
the  cardinal  line,  and  is  directed  toward  the  left  side  of  the 
shell.  The  apex  is  truncated,  and  the  opening  is  confluent 
with  the  area  below. 

The  cardinal  area  is  high,  forming  a  large  triangular  fissure 
which  is  apparently  not  closed  by  deltidial  plates.  The  beak 
of  the  dorsal  valve  is  depressed,  and  limited  by  a  slight  fur- 
row on  each  side.  No  lines  of  growth  are  visible,  but  the 
surface  is  somewhat  granulose,  as  in  many  small  shells  of 
other  species. 

The  principal  differences  to  be  noted  in  comparison  with 
the  adult  individuals  are  the  sub-circular  outline  of  the  shell, 
the  depressed  valves,  the  absence  of  a  median  fold,  and  the 
large  deltidial  area. 

Dictyonella  reticulata  Hall,  1868. 

(PLATE  XVII,  figures  11-13.) 

Eichwaldia  reticulata  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 

Nat.  Hist.,  p.  169,  pi.  26,  figs.  50-54,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  312,  pi.  26, 

figs.  50-54,  1882. 

Very  few  of  the  earlier  growth-stages  of  this  species  have 
been  observed,  and  these  show  but  comparatively  little  varia- 

*  James  Hall.    Bead  before  the  Albany  Institute,  March  18,  1879. 


336  STUDIES  IN  EVOLUTION 

tion  from  the  features  of  the  normal  adult.  On  Plate  XVII 
is  given  a  figure  of  the  youngest  example  found,  which  has 
a  length  and  width  of  3  mm.,  while  the  usual  adult  is  about 
16  X  16  mm.,  varying  in  relative  proportions  with  the  in- 
crease of  senile  obesity.  The  change  in  outline  during 
growth  is  from  sub-circular  to  sub-triangular,  and  in  earlier 
stages  the  ventral  fold  and  sinus  are  very  ill  defined.  The 
peculiar  triangular  exfoliation  of  the  shell  on  the  umbo  of 
the  ventral  valve  is  evidently  a  constant  feature  in  every 
stage  of  growth  after  the  shell  becomes  attached.  The 
nature  of  this  peculiarity  was  indicated  by  Billings  in  the 
original  diagnosis  of  the  genus  Eichwaldia  (Ann.  Rept.  Cana- 
dian Creol.  Survey,  1857-58),  and  was  demonstrated  more 
fully  in  Dictyonella  by  Professor  Hall,  in  the  Twentieth 
Report  on  the  Condition  of  the  New  York  State  Cabinet  of 
Natural  History  (pp.  274-278,  1867).  This  area  is  underlaid 
by  an  internal  shelf  or  diaphragm  attached  along  its  lateral 
margins,  and  having  fully  or  rather  more  than  the  width  of 
the  median  sinus.  Through  the  space  thus  left  between  the 
shell  and  the  internal  diaphragm,  communication  is  afforded 
with  the  outside  world.  Mr.  John  Young  has  called  atten- 
tion to  the  fact  that  in  D.  Capewelli  the  margins  of  the 
external  reticulated  layer  of  the  shell  about  the  umbonal  bare 
spot  are  rough  and  ragged,  the  superficial  hexagonal  cells 
being  without  finish  along  these  edges,  suggesting  therefrom 
that  the  animal  was  attached  to  marine  objects  by  the  sub- 
stance of  the  shell,  and  afterward  broken  away  from  its 
attachment.  (See  Davidson,  General  Summary, .  pp.  355, 
356.)  It  is  true  that  the  anterior  edge  of  this  area  may  be 
rough  and  uneven,  but  the  lateral  edges  appear  invariably 
straight  and  diverge  at  an  essentially  constant  angle.  The 
latter  represent  the  lines  of  attachment  of  the  internal  plate 
to  the  interior  of  the  valve,  and  if  the  shell  has  been  broken 
in  detachment  from  foreign  bodies,  the  fracture  in  these 
directions  has  been  guided  by  these  lines,  but  on  the  unsup- 
ported anterior  margin  it  has  been  rough  and  irregular. 
Upon  the  hinge-line  of  the  ventral  valve  there  exists  no 


DEVELOPMENT  OF  SOME  SILURIAN  RRACHIOPODA     337 

aperture  for  the  protrusion  of  the  pedicle;  by  the  peculiar 
development  of  the  articulating  processes  of  both  valves,  the 
entire  cardinal  margin  is  closed,  and  therefore  the  passage 
between  the  internal  plate  and  the  surface  of  the  valve  may 
have  been  for  the  use  of  this  organ;  or,  it  may  be  suggested 
that  as  this  space  is  rather  too  narrow  and  explanate  for  such 
a  purpose,  Dictyonella  may  have  been  attached  by  the  sub- 
stance of  the  shell,  the  internal  shelf  acting  as  a  support  to 
the  strain  upon  the  umbo  and  as  a  protection  to  the  animal 
in  case  the  shell  were  broken  from  its  attachment. 

Anastrophia  internascens  Hall,  1879. 
(PLATE  XVII,  figures  14-16.) 

Hall.     Twenty-eighth  Ann.  Kept.  K  Y.  State  Mus.  Nat.  Hist., 

p.  168,  pi.  26,  figs.  41-49,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  311,  pi.  26, 

figs.  41-49,  1882. 

In  tracing  the  development  (  of  this  species  the  principal 
feature  to  be  noticed  is  that  the  elemental  shell  conforms  with 
the  type  of  an  ordinary  brachiopod,  such  as  Camarotoechia ; 
that  is,  the  dorsal  valve,  although  somewhat  the  more  con- 
vex, is  smaller  than  the  opposite  valve,  while  in  the  mature 
state  the  dorsal  valve  is  considerably  larger  and  projects 
beyond  the  beak  of  the  ventral  valve.  It  is  the  development 
of  this  character  which  constitutes  the  most  conspicuous 
change  in  the  shell  in  its  growth  from  the  young  to  the  fully 
mature  condition. 

Specific  Characters. 

Mature  Form  (Plate  XVII,  figures  15,  16,  16  a).  —  Shell 
ventricose.  Outline  transversely  sub-elliptical,  sometimes 
nearly  as  long  as  wide. 

Ventral  valve  convex,  depressed  in  front,  forming  a  more 
or  le'ss  defined  sinus  which  carries  four  or  five  of  the  plica- 
tions; beak  short,  acute;  area  short,  broadly  triangular, 
usually  not  exposed. 

22 


STUDIES  IN  EVOLUTION 

Dorsal  valve  gibbous,  with  the  central  portion  elevated, 
frequently  presenting  a  broad  undefined  median  fold;  beak 
incurved  under  the  beak  of  the  opposite  valve;  umbo 
prominent. 

Surface  marked  by  about  fifteen  strong,  simple,  elevated, 
rounded  or  angular  plications  on  the  body  of  the  shell,  and 
smaller  bifurcating  plications  on  the  latera.  Occasionally 
intercalated  plications  are  present  on  the  middle  of  the 
valves.  The  plications  are  crossed  by  fine  arching  striae  of 
growth,  which  are  sometimes  aggregated,  forming  conspicuous 
concentric  lines  or  varices  of  growth. 

Mature  shells  measure  from  11  to  17  mm.  in  length,  and 
from  12  to  19  mm.  in  width.  The  depth  of  the  conjoined 
valves  varies  from  9  to  12  mm. 

incipient  Form  (Plate  XVII,  figures  14,  14  a).  — The  small- 
est shell  observed  has  a  length  of  2  mm.  and  a  width  of 
2.25  mm.  The  dorsal  valve  is  slightly  more  convex  than 
the  ventral,  and  is  a  little  shorter.  Eight  rounded  plications 
are  shown,  five  of  which  extend  to  the  umbo  of  the  valve. 
A  short  plication  is  intercalated  in  the  middle,  and  there  is 
also  a  short  one  on  each  side  of  the  valve.  Ventral  beak  small 
and  elevated,  with  a  broad,  triangular,  open  area  below. 

Developmental  Changes. 

The  series  of  specimens  selected  to  represent  the  develop- 
ment of  this  species  contains  fifteen  normal  individuals,  vary- 
ing from  a  length  of  2  mm.  to  a  length  of  17  mm.  The 
proportions  of  length  and  width  remain  nearly  constant 
throughout,  the  width  being  somewhat  the  greater. 

In  the  smallest  specimen  the  depth  of  both  valves  is  less 
than  one-half  the  length  of  the  shell.  This  relation  grad- 
ually changes  as  the  shell  becomes  larger  and  more  convex, 
until,  in  mature  individuals,  the  depth  is  equal  to  three- 
fourths  or  four-fifths  the  length  of  the  shell,  and  in  extremely 
obese  specimens  this  ratio  is  often  exceeded. 

The  dorsal  valve  is  more  convex  than  the  opposite  valve, 
in  all  the  stages  of  growth  which  have  been  observed, 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     339 

although  in  the  elemental  shell  the  difference  is  scarcely  per- 
ceptible, while  in  the  mature  form  it  is  a  conspicuous  feature. 
This  valve  is  also  shorter  than  the  ventral  in  specimens  up 
to  a  length  of  about  7  mm.  From  7  to  12  mm.,  both  valves 
are  of  nearly  equal  length.  Further  growth  causes  the  umbo 
of  the  dorsal  valve  to  protrude  beyond  the  beak  of  the  oppo- 
site valve,  and  the  beak  is  incurved  and  penetrates  the  area. 
It  seems  evident  that  if  the  true  initial  shell  were  studied 
the  dorsal  valve  would  be  found  not  only  smaller  but  less 
convex  than  the  opposite  valve. 

The  fold  begins  to  be  apparent  in  individuals  having  a 
length  of  about  10  mm.,  and  is  expressed  by  the  arching  of 
the  anterior  margin.  It  does  not  sufficiently  develop  to 
become  a  characteristic  feature,  and  is  more  or  less  undefined, 
even  in  many  full-grown  specimens. 

The  plications  increase  both  by  bifurcation  and  interstitial 
addition.  The  smallest  number  observed  is  eight,  and  this 
is  gradually  increased  with  the  growth  of  the  shell,  until 
there  are  about  fifteen  principal  plications  on  the  body  of 
the  shell,  and  several  smaller  ones  just  below  the  cardinal 
extremities.  The  concentric  striae  are  not  often  preserved, 
and  the  plications  therefore  form  the  only  conspicuous  char- 
acter of  the  surface  ornamentation. 

The  delthyrium  or  triangular  opening  becomes  completely 
filled  by  the  incurved  beak  of  the  dorsal  valve. 

Camarotoechia  acinus  Hall,  1863. 

(PLATE  XVIII,  figures  9-11.) 

Rhynchonella  acinus  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 

Nat.  Hist.,  p.  306,  pi.  26,  figs.  7-11,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  306,  pi.  26, 

figs.  7-11,  1882. 

Were  Camarotoechia  acinus  a  rare  species,  it  might  readily 
be  confounded  with  the  variety  of  C.  indianensis  which  bears 
but  a  single  plication  in  the  ventral  sinus.  It  appears,  how- 
ever, to  have  been  very  prolific,  and  its  abundance  serves  to 
emphasize  its  specific  independence.  The  liability  to  con- 


340  STUDIES  IN  EVOLUTION 

fuse  it  with  any  of  the  associated  species  arises  only  among 
forms  of  immature  growth.  Beginning  with  a  shell  which  is 
apparently  in  the  actual  initial  stage,  measuring  1.2  x  .8  mm., 
the  present  series  is  very  evenly  consecutive  up  to  maturity, 
when  the  average  dimensions  are  8x6  mm. 

Specific  Characters. 

Mature  Form  (Plate  XVIII,  figures  11-11  b).  —Shell  small, 
longitudinally  ovate,  sub-attenuate  toward  the  beak,  and 
truncate  in  front.  Cardinal  margins  long  and  rapidly  slop- 
ing, extending  more  than  half-way  across  the  shell;  sides 
flattened,  slightly  excavate.  Valves  sub-equally  convex. 

Ventral  valve  full  and  rotund  on  the  umbonal  region,  flat- 
tened at  about  the  middle,  thenceforward  sinuate ;  beak  in- 
curved, but  not  procumbent;  foramen  generally  concealed, 
or  when  slightly  exposed,  elongate  or  sub-triangular. 

Dorsal  valve  more  flattened  in  the  umbonal  region  and  in 
the  middle,  whence  a  low  fold  proceeds  to  the  margin. 

Surface  marked  by  low  rounded  plications.  The  ventral 
sinus  bears  a  single  plication  which  is  generally  faint,  often 
nearly  obsolete.  On  each  side  of  the  sinus  are  four  plica- 
tions, those  abutting  on  the  cardinal  margins  being  indis- 
tinct. On  the  dorsal  valve  the  low,  flattened  fold  bears  two 
plications  which  are  the  strongest  upon  the  shell;  these  are 
accompanied  by  three  plications  on  each  latus,  making  the 
whole  number  on  this  valve  eight.  No  concentric  growth- 
lines  are  apparent.  Average  dimensions  8x6  mm. 

Variations  from  the  Normal  Adult.  —  Two  plications  some- 
times occur  in  the  sinus,  and  in  such  cases  they  are  each 
stronger  than  the  single  sinal  plication  in  the  normal  adult. 
The  addition  of  the  plication  to  the  sinus  increases  the  num- 
ber in  the  fold  to  three,  and  the  total  number  of  plications  on 
the  shell  by  two. 

Initial  Shell  (compare  Plate  XVIII,  figures  9,  9  a,  95).  — 
Two  individuals,  one  measuring  1.2x8  mm.,  the  other 
1.4  X  9  mm.,  apparently  indicate  the  initial  stages  in  the 
growth  of  this  shell.  Neither  of  these  examples  has  served 
well  for  illustration,  on  account  of  the  lack  of  well-defined 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     341 

details,  but  they  may  be  described  as  follows:  Attenuate, 
sub-spatulate.  Ventral  valve  with  erect,  straight  beak ,  car- 
dinal area  high,  convex,  with  a  prominent  dorsum.  Dorsal 
valve  flattened  or  slightly  sinuate.  Features  of  the  cardinal 
area  not  discernible;  from  analogy,  the  foramen  would  be 
triangular  and  unobstructed.  In  figures  9-9  6,  which  show  a 
secondary  stage  of  growth  in  the  shell,  the  portion  included 
within  the  first  growth-line  will  represent  very  well  the  char- 
acters of  the  primitive  shell. 

General  Developmental  Characters. 

The  gradual  incurvature  of  the  beak  and  consequent  con- 
cealment of  the  ventral  foramen  may  be  assumed  from  the 
foregoing.  It  harmonizes  with  the  associated  species  of  the 
same  genus  in  the  slight  variation  in  the  form  and  propor- 
tions of  the  foramen  in  consecutive  stages  of  growth,  as  well 
as  in  the  reversal  of  the  embryonic  fold  and  sinus  to  the 
mature  sinus  and  fold.  The  plications  of  the  latera  seem  to 
appear  simultaneously  after  the  first  varix,  as  shown  in  the 
figures  referred  to,  and  their  number  does  not  change  mate- 
rially until  maturity.  The  embryonic  sulcus  on  the  dorsal 
valve,  correlate  with  the  ventral  dorsum  in  the  primitive 
stage,  is  continued  at  maturity  into  the  median  sulcus  sepa- 
rating the  two  plications  of  the  dorsal  fold. 

Camarotcechia  neglecta  Hall,  1852. 
(PLATE  XVIII,  figures  3,  6-8.) 

Rhynchonella  neglecta  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 

Nat.  Hist,  p.  162,  pi.  26,  figs.  1-6,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  305,  pi.  26, 

figs.  1-6 ;  pi.  27,  fig.  3,  1882. 

For  a  species  so  abundant  as  this  in  the  Waldron  fauna, 
the  diagnostic  features  are  retained  with  unusual  persistence 
within  very  narrow  limitations.  Unlike  its  associate,  0.  in- 
dianensis,  which  it  almost  equals  in  numerical  representa- 
tion, there  are  no  well-established  and  perduring  variations 


342  STUDIES  IN  EVOLUTION 

from  the  normal  adult  form,  and  these  observations  are  there- 
fore limited  to  an  essentially  unvarying  phase. 

Specific  Characters. 

Mature  Form  (Plate  XVIII,  figures  8,  8  a).  —  Shell  small, 
transversely  sub-ovate ;  unibo  scarcely  prominent.  Cardinal 
slopes  long  and  flattened,  rounding  to  the  anterior  margin 
which  is  nearly  straight. 

Ventral  valve  with  the  umbo  elevated  and  slightly  incurved 
at  the  tip,  overhanging  an  elongate  sub-triangular  foramen. 
Umbonal  region  slightly  convex,  the  convexity  extending  for 
one -third  the  length  of  the  shell;  thence  forward  the  shell  is 
rapidly  depressed  medially  to  form  a  deep  sinus,  which  makes 
a  high  quadrangular  extension  on  the  margin ;  lateral  portions 
depressed. 

Dorsal  valve  with  the  umbo  low  and  inconspicuous;  apex 
concealed  within  the  foramen  of  the  opposite  valve.  The 
shell  becomes  rapidly  elevated  medially  to  form  the  fold,  the 
latera  being  full  and  convex. 

Surface  covered  with  regular,  sharp,  and  prominent  plica- 
tions, which  do  not  vary  in  number  at  normal  maturity,  and 
which,  in  the  growth  of  the  shell,  are  increased  only  from  the 
cardinal  margins.  Of  these  plications,  the  fold  bears  four, 
the  sinus,  therefore,  three,  and  each  of  the  latera  five,  those 
nearest  the  cardinal  margins  being  obscure.  This  makes  in 
all  for  the  ventral  valve,  thirteen, -and  for  the  dorsal,  four- 
teen plications.  Dimensions  of  an  average  example,  length, 
width,  and  depth,  9,  8,  and  5  mm. 

\  single  individual  presents  the  only  important  abnormal- 
ity noticed;  namely,  a  failure  to  produce  the  requisite  plica- 
tions upon  the  latera,  the  dorsal  valve  bearing  but  five,  and  the 
ventral,  six.  Of  these,  three  are  on  the  fold,  two  in  the 
sinus.  It  is  interesting  to  notice  that  in  the  umbonal  region 
the  normal  number  of  plications  had  been  formed  in  their 
regular  arrangement;  their  disappearance  on  the  latera  and 
irregular  disposition  in  fold  and  sinus  took  place  abruptly 
upon  the  completion  of  a  growth -line  2  mm.  from  the  apex. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     343 

This  is  a  marked  instance  of  reversion  after  the  assumption 
of  certain  adult  features. 

incipient  Form  (Plate  XVIII,  figures  6,  6  a).  —  The  ex- 
ample with  which  the  present  series  opens  measures  .75  x 
.5  mm.  It  is  elongate  sub- triangular,  with  the  ventral  beak 
elevated  and  erect,  the  cardinal  margins  sloping  for  two-thirds 
the  length  of  the  shell ;  foramen  triangular,  slightly,  if  at  all, 
encroaching  upon  the  apex,  without  deltidial  plates,  margins 
thin ;  dorsal  beak  rounded,  inconspicuous.  At  one-third  the 
distance  from  the  apex  to  the  anterior  margin,  fine  thread- 
like plications  appear,  four  upon  the  dorsal,  and  three  (five  ?) 
upon  the  ventral  valve.  The  median  sulcus  on  the  dorsal 
valve  is  broader  and  deeper  than  any  other,  forming  the  em- 
bryonal sinus,  and  is  accompanied  by  a  correlatively  strong 
plication  on  the  opposite  shell. 

Developmental  Variations. 

General  Form  and  Outline.  -?-  The  form  of  the  shell  varies 
from  dimensions  in  which  the  length  is  one-quarter  greater 
than  the  width,  to  those  of  maturity  when  the  width  is 
slightly  greater  than  the  length.  The  depressed,  sub-spatu- 
late  embryo  eventually  becomes  convex  and  deep.  The  em- 
bryonal sinus  and  fold  on  the  dorsal  and  ventral  valves, 
respectively,  are  never  so  prominent  as  in  0.  indianensis,  and 
soon  become  lost,  the  former  in  a  sulcus,  and  the  latter  in  one 
or  more  sulci  upon  the  reversed  fold  and  sinus  of  maturity. 

Beak  and  Foramen.  —  The  erect  and  acute  beak  of  the  ele- 
mentary stages  of  growth  becomes,  at  maturity,  but  slightly 
incurved,  and  never  procumbent  on  the  dorsal  umbo.  The 
foramen,  at  the  outset  triangular,  subsequently  has  its  mar- 
gins thickened,  and  develops  small  and  obscure  deltidial  plates 
at  its  base,  which  at  maturity  leave  the  foramen  elongate  and 
not  circular.  In  respect  to  these  features,  the  development 
of  the  species  is  identical  with  that  of  0.  indianensis. 

Plications.  —  In  the  first  observed  stadium  only  the  um- 
bonal  area  is  smooth,  and  from  the  analogy  of  C.  indianensis 


344  STUDIES  IN  EVOLUTION 

it  would  appear  that  the  initial  growth-stage  is  still  wanting. 
At  a  size  of  2.5  X  2  mm.,  the  number  of  plications  has 
increased  from  four  to  ten  on  the  dorsal,  and  from  three 
(five  ?)  to  eleven  on  the  ventral  valve ;  and  this,  added  to  a 
pair  of  extremely  obscure  plications  near  the  cardinal  mar- 
gins, is  the  normal  number  for  maturity. 

Camarotcechia  Whitii  Hall,  1863. 

(PLATE  XVIII,  figures  1,  2,  4,  5.) 

nhynchondla  Whitii  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 

Nat.  Hist.,  p.  164,  pi.  26,  figs.  23-33,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  307,  pi.  26, 

figs.  23-33,  1882. 

Like  Camarotoechia  neglecta,  this  species  is  subject  to  very 
slight  variations  at  maturity,  and  its  specific  expression  is 
well  marked,  but  a  certain  embarrassment  attends  the  first 
endeavor  to  separate  the  immature  individuals  from  those  of 
allied  species.  This,  however,  disappears  with  a  careful  eye 
properly  estimating  the  essential  characters  of  the  species. 
The  earliest  stage  of  growth  found  measures  2.75  mm.  in 
length  by  2  mm.  in  breadth,  and  from  this  size  upward 
to  that  of  13  x  13  mm.  all  variations  are  present. 

Specific  Characters. 

Mature  Form  (Plate  XVIII,  figures  2,  2  a,  2  b).  —  Shell 
transversely  sub-elliptical;  length  and  width  about  equal. 

Ventral  valve  shallow;  beak  high,  acute,  somewhat  atten- 
uate, with  the  apex  slightly  incurved,  but  not  concealing  the 
triangular  unclosed  foramen  which  reaches  entirely  across  the 
cardinal  area.  At  its  apex  the  foramen  encroaches  slightly 
upon  the  umbo,  and  is  narrowed  somewhat  toward  the  base 
by  the  imperfectly  developed  deltidial  plates.  A  median 
depression  makes  its  appearance  at  about  one-third  the  dis- 
tance from  the  umbo  to  the  anterior  margin,  and  soon 
develops  into  a  deep  sinus  with  sharply  sloping  sides. 

Dorsal  valve  deeper  and  more  gibbous ;  beak  inconspicuous, 


DEVELOPMENT   OF  SOME   SILURIAN  BRA CHIOPODA     345 

and  incurved  beneath  the  ventral  foramen.  A  strong  median 
fold  corresponds  in  development  with  the  median  sinus  of  the 
opposite  valve. 

Surface  marked  by  strong,  simple,  sub-angular  plications, 
invariably  two  upon  the  fold  and  one  in  the  sinus,  with  six 
on  each  of  the  latera,  making  thirteen  on  the  ventral  and 
fourteen  on  the  dorsal  valve.  Of  these  the  plications  near  the 
cardinal  margin  are  low  and  incipient,  but  the  full  number 
becomes  permanent  early  in  the  history  of  the  individual. 
Faint  concentric  growth-lines  are  sometimes  visible.  Dimen- 
sions of  average  adult  11  X  11  mm. 

Abnormalities  at  Maturity.  —  The  variations  from  the  nor- 
mal mature  form  are,  as  far  as  observed,  wholly  due  to  con- 
tinued internal  growth  after  individual  maturity  has  been 
attained,  and  this  is  to  be  regarded  as  the  concomitant  evi- 
dence of  senescence.  There  may  be  either  a  marginal  thicken- 
ing, which  gives  the  shell  a  truncate  appearance,  or  a  general 
internal  thickening,  making  the  shell  unusually  gibbous,  and 
forcing  the  ventral  beak  over  upon  the  dorsal  umbo. 

incipient  Form  (Plate  XVIII,  figures  1,  la).  —  The  young- 
est individual  observed  measures  2.75  X  2  mm. ;  outline  sub- 
ovate,  valves  regularly  rounded,  the  ventral  being  the  more 
convex.  Ventral  valve  with  an  erect,  straight  beak;  apex 
acute,  cardinal  margins  sloping  rapidly  forward,  and  slightly 
excavate.  Foramen  simple,  triangular,  free  from  deltidial 
plates,  encroaching  at  its  apex  slightly  upon  the  umbo ;  f or- 
aminal  margins  somewhat  thickened.  Dorsal  beak  erect  but 
inconspicuous,  full  and  rounded.  Dorsal  valve  depressed 
anteriorly  along  the  median  line,  this  depression  correspond- 
ing with  the  broad  and  low  dorsum  of  the  opposite  valve. 
Surface  of  each  valve  marked  by  eight  single,  rounded  plica- 
tions, which  extend  two-thirds  the  distance  from  the  anterior 
margin  of  the  beak,  leaving  the  circumbonal  area  smooth. 

Developmental  Variations. 

General  Form  and  Outline.  —  As  growth  advances,  the  de- 
velopment is  more  rapid  transversely  than  longitudinally, 


846  STUDIES  IN  EVOLUTION 

and,  consequently,  the  sub-ovate  incipient  shell  becomes,  at 
maturity,  broadly  transverse.  The  prominent  dorsum  of  the 
ventral  valve  in  the  embryo  is  manifest  at  maturity  only  in 
the  rounded  and  prominent  beak,  and  the  embryonal  sinus  in 
the  dorsal  valve  becomes  so  thoroughly  obsolete  at  maturity 
as  to  be  unnoticeable.  In  stages  of  development  between 
the  dimensions  3.5  x  3  mm.  and  6  x  5.5  mm.,  the  ventral 
valve  still  retains  a  slightly  greater  convexity,  but  the  ante- 
rior margin  is  entire. 

Beak  and  Foramen.  —  The  erect  and  straight  beak  of  the 
incipient  shell  becomes  slightly  incurved  toward  maturity, 
but  the  cardinal  area  remains  high,  exposing  the  triangular 
foramen  at  all  stages  of  growth.  Deltidial  plates  make  their 
appearance  early,  but  never  develop  sufficiently  to  meet  and 
enclose  the  pedicle-aperture,  a  feature  indicative  of  arrested 
development,  and  equally  true  of  the  other  members  of  the 
genus  here  discussed. 

Plications.  —  The  fact  that  the  eight  plications  on  each 
valve  of  the  incipient  shell  do  not  reach  the  umbones  indi- 
cates that  the  initial  shell  may  have  been  smooth,  as  it 
has  been  shown  to  be  in  R.  indianensis.  The  subsequent 
addition  of  plications  takes  place  slowly  and  from  the  cardinal 
margins. 

Camarotoechia  indianensis  Hall,  1863. 

(PLATE  XVII,  figures  17-28.) 

Rhynchonella  indianensis  Hall.  Twenty-eighth  Ann.  Kept.  N.  Y.  State 
Mus.  Nat.  Hist.,  p.  163,  pi.  26,  figs.  12-22,  1879. 

Hall.  Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  306,  pi.  26, 

figs.  12-22;  pi.  27,  figs.  4-6,  1882. 

Camarotoechia  indianensis  is,  beyond  a  doubt,  the  most 
prolific  species  in  the  rich  fauna  of  the  Waldron  beds,  and 
by  virtue  of  this  fact  it  has  been  possible  to  ascertain  the 
developmental  phases  through  illustrative  series  of  excep- 
tional completeness.  It  is  noteworthy  that  the  mature  shell 
of  this  Camarotcechia  presents  variations  from  the  adult  type, 
which  are  so  great  that  in  a  certain  sense  they  might  be 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     347 

regarded  as  passing  the  limitations  of  specific  identity;  how- 
ever, the  general  form  and  expression  of  the  shell  are  charac- 
teristic, so  that,  in  spite  of  these  variations,  no  confusion  with 
allied  species  of  the  same  fauna  can  arise,  nor  need  there 
be  any  hesitation  to  assign  to  the  different  forms  a  varietal 
significance  only.  Probably  ten  thousand  individuals  of  this 
species  have  passed  under  the  observation  of  the  writers,  and 
of  this  large  number  fully  one-half  have  been  immature  forms. 

Specific  Characters. 

Normal  Mature  Form;  containing  two  plications  in  the 
sinus  of  the  ventral  valve  (Plate  XVII,  figure  21).  —  Shell 
sub-triangular  or  broadly  ovate;  length  nearly  equal  to, 
sometimes  slightly  exceeding  the  width.  Umbo  prominent, 
sub-acute;  cardinal  slopes  extending  one-half  the  length  of 
the  shell,  and  flattened. 

Ventral  valve  depressed  convex,  rounded  at  the  beak ;  apex 
pointed  and  slightly  incurved,  exposing  beneath  it  the  elon- 
gate, narrow  foramen  and  the  .inconspicuous  deltidial  plates. 
Dorsum  for  the  first  one-third  the  length  of  the  shell  rounded, 
thence,  anteriorly,  gradually  becoming  depressed.  The  sinus 
thus  formed  bears  two  strong,  rounded  plications  which  are 
of  later  origin  than  the  pair  which  forms  its  lateral  boun- 
daries. The  latera  each  bear  three  plications  with  traces  of  a 
fourth,  making  eight  (ten)  on  the  entire  surface  of  the  valve. 

Dorsal  valve  somewhat  deeper  than  the  ventral,  flattened 
above,  depressed  near  the  beak  along  the  median  line  (em- 
bryonal sinus),  thenceforward  becoming  gradually  elevated 
into  a  fold  which  bears  three  strong  rounded  plications.  Four 
similar  plications  are  discernible  on  each  of  the  latera,  mak- 
ing in  all  eleven  plications  on  the  entire  valve.  Umbo  incon- 
spicuous, apex  concealed  within  the  foramen  of  the  opposite 
valve.  Concentric  growth-lines  obscure,  or  absent.  Average 
dimensions  12  x  12  mm. 

These  are  assumed  as  the  normal  characters  of  adult  growth 
on  account  of  the  great  predominance  of  specimens  bearing 
two  plications  in  the  ventral  sinus. 


348  STUDIES  IN  EVOLUTION 

Variations  from  the  Normal.  —  (A)  Forms  with  one  plication 
in  the  ventral  sinus.  This  variation  does  not  attain  quite  the 
size  of  the  average  normal  adult,  but  retains  the  same  pro- 
portion of  length  and  breadth  (size  10  x  10  mm.).  The 
surface  bears  ten  plications  on  the  dorsal  and  nine  on  the 
ventral  valve.  In  this  form  the  embryonal  sinus,  visible  on 
the  earlier  portion  of  the  dorsal  valve,  is  distinctly  continu- 
ous with  the  strong  sulcus  separating  the  two  plications  on 
the  fold  in  the  later  and  marginal  portions  of  the  valve. 
This  variation  is  not  of  uncommon  occurrence,  and  immature 
individuals  in  various  stages  of  development  prove  that  it  is 
a  well-established  genetic  difference,  and  not  merely  an  occa- 
sional monstrosity. 

(B)  Forms  with  three  plications  in  the  ventral  sinus.     The 
size  and  proportions  of  the  normal  are  retained  in  this  variety, 
but  the  shell  bears  usually  three,  sometimes  four  plications 
on  each  of  the  latera,  making  ten  (twelve)  plications  for  the 
dorsal,  and  nine  (eleven)  for  the  ventral  valve.     This  form  is 
of  comparatively  rare  occurrence,  and  is  not  often  noticed  in 
an  immature  stage  of  growth. 

(C)  Forms  with  four  plications  in  the  ventral  sinus.     This 
variation  is  met  with  very  infrequently,  but  two  individuals 
having  been  obtained.     While  agreeing  in  size  with  the  nor- 
mal adult,  the  crowding  of  the  sinus  with  plications  tends  to 
obliterate   both   it  and  the   fold  upon   the   opposite  valve. 
Both  individuals   show  the   interesting  fact  that  upon  the 
dorsal  valve  where  the  fold  bears   five   plications,  that  is, 
four  sulci,  the  embryonal  sinus  is  continuous  with  the  third  of 
these  sulci,  in  one  instance  numbering  from  the  right,  in  the 
other  numbering  from  the  left.     Of  the  five  plications  which 
are  thus  separated  into  groups  of  three  and  two,  it  is  notice- 
able that  the  outer  member  of  the  group  of  three  is  both  less 
elevated  and  shorter  than  any  other  upon  the  fold. 

Monstrous  Forms.  —  The  sole  evidence  of  monstrous  growth 
observed  is  an  asymmetrical  development  of  the  plications 
upon  the  dorsal  fold.  Examples  bearing  three  plications  upon 
the  fold,  in  rare  instances  have  one  of  the  plications  very 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     349 

large  and  two  quite  small,  making  one  broad  and  one 
narrow  sulcus  upon  the  fold.  The  phenomenon  may  be  due 
to  the  strongly  developed  tendency  of  the  embryonal  sinus  to 
maintain  its  continuity  with  a  median  sulcus  even  at  the  ex- 
pense of  the  symmetry  of  the  shell. 

Initial  SheU  (Plate  XVII,  figures  17,  17  a).  — The  initial 
shell  in  this  series  of  Camarotoechia  indianensis  measures  .65 
mm.  in  length  by  .54  mm.  in  width.  It  is  broadly  ovate  or 
sub-pyriform  in  aspect,  convex  posteriorly,  and  depressed 
toward  the  anterior  margin.  Ventral  valve  with  the  umbo 
prominent,  the  beak  elevated  and  erect,  with  the  apex  rounded ; 
cardinal  margins  rapidly  sloping.  Foramen  sub-triangular, 
apical  portion  broader  than  usual  in  the  incipient  stages  of 
plicate  shells ;  margins  not  thickened ;  deltidial  plates  absent. 
Dorsal  valve  with  a  rounded,  inconspicuous  beak.  Surface  of 
both  valves  quite  smooth.  A  median  depression  is  noticeable 
on  the  dorsal  valve  near  the  anterior  margin,  making  this 
margin  sinuate.  This  embryo  is  the  smallest  that  has  been 
found  for  any  of  the  series  of  Rhynchonellidse ;  and  not  only 
on  account  of  its  minuteness,  but  also  from  the  entire  absence 
of  plications  on  its  surface  and  from  the  elementary  character 
of  the  cardinal  area,  it  may  be  regarded  as  the  actual  elemental 
or  initial  shell. 

Developmental    Variations. 

General  Form  and  Outline.  —  The  adult  variations  from  the 
normal  noticed  above  seem  to  be  in  most  instances,  and  prob- 
ably would  prove  to  be  in  all,  preceded  by  well-defined 
embryonic  series  leading  up  to  them.  This  must  be  the 
case,  as  the  character  of  these  variations,  i.  e.  variation  in 
the  number  of  plications  on  the  median  portions  of  the  shell, 
is  such  that  they  cannot  be  assumed  after  the  attainment  of 
the  adult  condition,  as  is  possible  in  certain  other  forms  of 
variation.  But  it  is  not  to  be  assumed  that  the  conformation 
of  the  embryo  which  eventually  produces  any  of  these  results 
manifests  them  in  the  earliest  stages  of  the  growth  of  the 
shell ;  rather,  that  the  shells,  under  whatsoever  variations  at 


350  STUDIES  IN  EVOLUTION 

maturity,  all  have  the  same  unspecialized  starting-point. 
Hence  the  fact  that  some  of  these  variations  have  not  shown 
a  complete  series  of  immature  stages  must  be  due  to  the 
insufficiency  of  the  material,  rich  as  it  has  been. 

Limiting  these  considerations  now  to  the  normal  form  with 
two  plications  in  the  ventral  sinus,  it  may  be  seen  that  the 
initial  shell  is  smooth,  and  obcordate  in  outline,  with  beak  erect, 
while  the  mature  shell  is  strongly  plicate,  strongly  ovate,  and 
with  the  beak  sharply  incurved.  The  transition  from  one 
extreme  to  the  other  is  through  stages  of  growth  between  the 
limits  .65  x.54  mm.  (initial)  and  12  X  12  mm.  (average  adult) 
In  growth-stages  below  7x5  mm.  dimensions,  the  shell  is  very 
depressed-convex,  the  dorsal  valve  up  to  about  this  point 
retaining  a  low,  broad,  median  depression,  accompanied  by  a 
similarly  low  and  broad  median  elevation  on  the  opposite 
valve.  It  is  not  always  possible  to  determine  with  accuracy 
how  many  plications  are  carried  by  this  embryonic  fold  and 
sinus,  on  account  of  not  being  well  limited;  but  their 
eventual  reversion,  in  the  adult  shell,  into  sinus  and  fold 
respectively,  marks  the  feature  as  an  interesting  one,  to 
which  attention  is  called  more  at  length  in  the  description 
of  the  species  Rhynchotreta  cuneata  and  Atrypa  reticularis. 
Rare  instances  occur  of  individuals  assuming  all  the  characters 
of  maturity  before  attaining  a  length  of  6  mm.,  and  from 
this  point  up  to  the  normal  size  for  adult  growth,  mature 
dwarfs  are  frequently  found. 

Beak.  —  In  the  initial  shell  the  beak  of  the  dorsal  valve  is 
rounded  and  inconspicuous,  and  so  remains  in  all  stages  of 
growth.  In  the  opposite  valve  the  beak  is  at  first  high, 
erect  but  not  acute,  the  cardinal  margins  sloping  abruptly ; 
and  with  increasing  age  the  beak  becomes  fuller,  more  and 
more  incurved  at  the  apex,  but  is  never  closely  procumbent 
upon  the  dorsal  umbo,  as  is  the  case  at  maturity  with  most  of 
the  plicate  species  here  described. 

Foramen.  —  At  the  outset  the  pedicle-aperture  is  narrowly 
sub- triangular,  reaching  to  and  encroaching  upon  the  apex, 
free  of  deltidial  plates  and  with  the  lateral  margins  unthick- 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     351 

ened,  i.  e.  elemental  in  every  respect.  In  the  second  stage  of 
growth  (after  the  appearance  of  plications  on  the  surface, 
dimensions  1.5  X  1.1  mm.),  the  apertural  margins  have  become 
thickened,  and  directly  thereafter  the  deltidial  plates  begin 
to  develop,  gradually  narrowing  the  aperture  at  the  base. 
The  symphysis  of  these  plates  with  the  valve  is  marked  by 
distinctly  elevated  lines.  In  maturity  the  deltidial  plates 
have  developed  sufficiently  to  close  completely  the  lower 
part  of  the  aperture,  coming  together  behind  the  beak  of  the 
dorsal  valve,  and  giving  to  the  foramen  an  elliptical  outline 
constricted  toward  the  apex,  where  it  encroaches  upon  the 
umbo.  The  fact  that  the  development  of  the  foramen  is 
thus  interrupted  before  it  reaches  the  circular  outline  normal 
to  the  adult  of  most  Paleozoic  species  indicates  an  embryonic 
character  in  the  adult,  and  therefore  a  subordinate  taxo- 
nomic  position  for  the  species. 

Plications.  —  These  appear  only  after  the  first  stage  of 
growth  is  passed  and  after  the  first  growth-line  has  been 
formed.  As  in  Homoeospira  evax,  they  appear  over  the  entire 
surface  of  the  shell  below  the  growth-line  all  at  once,  and 
from  this  stage  onward  to  maturity  no  increase  is  made  in 
the  number,  except  by  intercalation  along  the  margin  of  the 
fold  and  sinus. 

Rhynchotreta  cuneata  Dalman,  1827, 
var.  americana  Hall,  1879. 

(PLATE  XVIII,  figures  12-22.) 

Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  167,  pi.  25,  figs.  29-38,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  310, 

pi.  25,  figs.  29-38,  1882. 

The  individuals  of  this  species  do  not  so  readily  separate 
into  three  groups  of  long,  normal,  and  broad  forms,  as  do 
those  of  Camarotoechia  neglecta,  Homoeospira  evax,  Whitfieldella 
nitida,  and  others.  This  seems  to  be  due  to  the  uniformity 
in  the  number  of  plications,  and  also  in  the  number  carried 


352  STUDIES  IN  EVOLUTION 

on  the  fold  and  sinus.  The  long  and  broad  varieties  do 
exist,  however,  but  are  of  such  infrequency  as  to  suggest 
that  they  are  not  genetic  variations  from  the  typical  form 
occurring  in  this  locality. 

The  specimens  from  the  Wenlock  shales  of  Dudley  show 
a  considerable  variation  from  their  American  congeners  in 
having  more  numerous  plications,  of  which  a  greater  number 
is  raised  on  the  dorsal  fold  and  depressed  in  the  ventral 
sinus.  In  other  respects  it  is  believed  that  the  description 
here  given  of  the  development  of  the  shell  will  apply  to  the 
British  form. 

Rhynchotreta  cuneata,  although  considered  as  abundant  in 
the  mature  state,  does  not  approach,  in  the  number  of  young 
specimens,  Camarotcechia  Whitii,  C.  neglecta,  0.  indianensis, 
Homceospira  evax,  Whitfieldella  nitida,  Spirifer  crispus,  jSp. 
crispus,  var.  simplex,  Atrypa  reticularis,  Rliipidomella  hybrida, 
and  Dalmanella  elegantula.  The  entire  number  of  young  in- 
dividuals examined  is  about  one  hundred  and  fifty,  ranging 
in  size  from  .8  to  1.5  mm.  in  length.  The  mature  forms 
average  about  17  mm.  in  length. 

In  the  several  series  selected  from  the  material  at  hand,  it 
is  evident  that  the  shell  assumed  the  characters  and  form  of 
maturity  when  reaching  a  length  of  about  10  mm.  At  this 
period  of  growth  the  fold  of  the  dorsal  valve  becomes  ele- 
vated, and  the  sinus  of  the  ventral  valve  depressed  (figure  15, 
Plate  XVIII).  Previous  to  this  stage,  the  dorsal  valve  is 
depressed  and  transversely  concave,  and  the  plications  of  the 
opposite  valve  are  raised  along  the  median  line  of  the  shell. 


Specific  Characters. 

Mature  Form  (Plate  XVIII,  figures  14,  14 a,  145,  22,  22  a). 
—  Shell  triangular,  cuneiform,  widest  across  the  pallial  region. 
Length  equal  to  about  twice  the  depth  of  the  valves.  Beaks 
compressed  laterally,  attenuate  and  pointed. 

Ventral  valve  moderately  convex,  sub-angular  along  the 
latera,  marked  by  a  deep  sinus,  which  commences  near  the 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     353 

middle  of  the  length,  and  becomes  very  marked  in  front, 
depressing  three  plications,  of  which  the  middle  one  is  de- 
truded more  than  the  others. 

Dorsal  valve  convex,  gibbous  in  the  posterior  part,  with 
the  latera  elevated  and  sub-angular;  marked  in  front  by  a 
prominent  fold  which  begins  near  the  beak  as  a  depression 
carrying  four  plications,  of  which  the  two  central  ones  are 
usually  much  more  elevated  than  the  other  pair. 

Area  high,  closed  by  two  triangular  deltidial  plates.  Per- 
foration of  the  ventral  beak  ovate,  truncating  the  apex,  and 
limited  below  by  the  deltidial  plates. 

Surface  marked  by  from  eight  to  ten  strong,  angular  plica- 
tions, which  are  crossed  by  very  fine,  regular,  sharp,  concen- 
tric striae.  Mature  specimens  usually  measure  from  10  to 
17  mm.  in  length. 

incipient  Form  (Plate  XVIII,  figures  12,  12  a).  —  The 
youngest  shell  detected  has  a  length  of  1.5  mm.,  is  flattened, 
and  nearly  circular  in  outline.  The  dorsal  valve  is  depressed 
in  the  middle  and  carries  four  plications.  The  beak  of  the 
ventral  valve  is  broadly  triangular,  exsert,  and  elevated,  with 
a  triangular,  open  area  without  deltidial  plates. 

Developmental  Changes. 

Contour.  —  In  the  earliest  stages  yet  noticed  the  shell  is 
nearly  circular.  At  a  length  of  2  mm.  it  is  broadly  oval,  and 
at  2.5  mm.  it  is  ovate.  The  beak  in  the  next  advanced  stage 
is  more  elongate,  and  when  the  length  of  4  mm.  is  reached, 
the  shell  has  a  decidedly  triangular  or  cuneate  form,  which 
becomes  more  pronounced  up  to  maturity.  All  the  young 
and  adolescent  shells  are  depressed,  the  characteristic  fulness 
of  the  valves  not  being  developed  until  after  the  assumption 
of  the  features  of  maturity,  and  when  the  shell  approaches  its 
normal  size. 

Fold  and  Sinus.  —  The  smallest  individual  shows  a  slight 
depression  in  the  dorsal  valve,  co-existing  with  the  plica- 
tions, beginning  about  one-fifth  the  length  of  the  shell  in 
front  of  the  beak,  widening  rapidly,  and  becoming  more 

23 


354  STUDIES  IN  EVOLUTION 

defined  upon  approaching  the  margin.  The  latera  are  nearly 
flat.  The  depression,  or  sinus,  becomes  more  pronounced 
with  the  advance  in  growth,  until  a  length  of  4.5  mm.  is 
attained.  After  this  period  the  four  bottom  plications  grad- 
ually elevate,  the  sinus  grows  shallower,  and  the  front  margin 
of  the  conjoined  valves  becomes  nearly  straight.  Upon  reach- 
ing a  length  of  9  mm.,  the  two  central  plications  are  suffi- 
ciently elevated  to  define  the  fold,  which  is  hereafter  the 
principal  feature  of  the  dorsal  valve.  The  development  from 
this  point  to  full-grown  individuals  is  principally  directed  to 
reaching  a  maximum  prominence  in  the  fold,  and  increasing 
the  shell  by  increment  on  the  lateral  margins  of  the  valves. 

The  development  of  a  sinus  in  the  dorsal  valve,  its  subse- 
quent obliteration,  and  the  final  elevation  of  the  plications 
into  a  strong  median  fold  are  shown  in  figure  15,  1-10  of 
Plate  XVIII,  in  which  the  undulating  lines  represent  the 
anterior  junction  of  the  valves. 

Beak.  —  The  apex  of  the  dorsal  valve  is  strong  and  pointed, 
and  is  visible  in  all  specimens  up  to  a  length  of  about  14  mm. 
After  this  stage  the  shell  becomes  obese,  and  the  consequent 
greater  inclination  of  the  beak  forces  it  into  the  foraminal 
cavity,  where  it  becomes  hidden  by  the  deltidial  plates. 

The  ventral  valve  is  uniformly  convex  in  all  incipient 
specimens.  The  sinus  develops  at  the  same  period,  and  in 
conformity  with  the  fold  of  the  opposite  valve.  The  beak  of 
the  initial  shell  is  broadly  triangular,  perforate  at  the  apex, 
and  directed  outward.  It  gradually  becomes  narrower  and 
less  oblique  with  advancing  growth,  and  lies  in  the  axis  of 
the  shell  in  full-grown  specimens.  The  initial  perforation  is 
a  small  truncation  of  the  beak,  confluent  with  the  open  area 
below.  (See  figure  16,  Plate  XVIII.) 

Surface  Ornaments.  —  The  prevailing  number  of  plications 
is  eight,  although  it  varies  from  seven  to  ten  in  some  speci- 
mens. The  entire  number  appears  at  an  early  period  of 
growth,  and  in  this  respect  the  species  offers  a  marked  differ- 
ence from  some  of  the  forms  of  Camarotoechia  already  con- 
sidered, in  which  the  plications  increase  by  pairs.  In  a 


DEVELOPMENT  OF  SOME   SILURIAN  BRACHIOPODA     355 

specimen  1.5  mm.  long,  they  first  appear  at  about  one-fifth 
the  length  of  the  shell  from  the  beak.  Four  plications  are 
included  in  the  depression  of  the  dorsal  valve  in  the  incipient 
stages,  and  the  two  central  ones  finally  become  elevated, 
forming  the  fold  in  the  full-grown  shell.  Upon  approaching 
maturity,  three  of  the  plications  in  the  ventral  valve  are 
depressed,  the  middle  one  ultimately  much  more  than  the 
others,  forming  the  single  strong  plication  at  the  bottom  of 
the  sinus. 

No  concentric  striae  are  shown  on  the  initial  shell  of  this 
series.  In  a  specimen  3  mm.  in  length,  these  begin  to  develop 
over  the  outer  third  of  the  surface,  as  shown  in  figure  13, 
Plate  XVIII. 

Cardinal  Area.  —  The  foramen  is  at  first  a  broad  triangular 
opening,  wider  than  high,  with  sharp  margins,  and  truncat- 
ing the  beak  of  the  ventral  valve.  The  lateral  margins  are 
thickened  in  a  specimen  3  mm.  in  length  (Plate  XVIII, 
figure  13),  and  the  height  and  width  of  the  area  are  equal. 
These  proportions  of  height  and  width  are  preserved  to 
maturity,  although  in  some  specimens  the  area  is  higher  than 
wide.  No  deltidial  plates  have  as  yet  appeared,  but  in  the 
next  stage,  including  individuals  having  a  length  of  4.5  mm., 
there  are  two  narrow  deltidial  plates  developed  from  the 
sides  of  the  foramen  (Plate  XVIII,  figure  18).  A  specimen 
5  mm.  in  length  shows  the  still  further  increase  in  the  size 
of  these  plates,  although  they  do  not  come  in  contact,  but 
leave  an  oval  opening  extending  from  the  ventral  beak  down 
to  the  beak  of  the  dorsal  valve  (Plate  XVIII,  figure  19). 
The  increase  in  the  growth  of  the  deltidial  plates  along  their 
inner  margins  brings  them  in  contact  under  the  dorsal  beak, 
in  specimens  having  a  length  of  about  7  mm.  (Plate  XVIII, 
figure  20).  Further  growth  truncates  their  inner  angles, 
thus  shortening  the  deltidial  opening.  In  individuals  about 
12  mm.  long  (figure  21),  the  opening  extends  but  little  more 
than  half  the  length  of  the  area,  and  the  lower  margin  of  the 
opening  is  thickened  and  slightly  deflected.  Fully  matured 
forms,  having  a  length  of  from  15  to  17  mm.,  have  a  perfora- 


356  STUDIES  IN  EVOLUTION 

tion  less  than  one-half  the  height  of  the  area,  which  truncates 
the  beak  more  strongly  than  in  younger  shells,  and  the  del- 
tidial  plates  show  a  defined  thickened  area  below  the  per- 
foration, often  extending  to  the  dorsal  beak  (Plate  XVIII, 
figure  22). 

Variations.  —  As  already  stated,  the  elongate  and  broadly 
flabellate  shells  appear  to  be,  in  this  species,  neither  common 
nor  genetic  variations.  Among  the  extraordinary  develop- 
ments are  specimens  with  duplicate  plications  in  the  sinus, 
and  one  showing  but  seven  plications  on  the  shell.  Another 
individual  has  the  initial  shell  strongly  defined  by  a  varix  of 
growth,  and  shows  on  this  portion  ten  plications,  but  in  the 
subsequent  growth  only  eight  plications  are  continued,  these 
alternating  at  the  varix  with  those  of  the  embryonic  shell. 

Atrypa  reticularis  Linnaeus,  1767. 

(PLATE  XX,  figures  12-20.) 

Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat.  Hist., 

p.  162,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  304,  1882. 

The  abundance  of  this  well-known  species  at  Waldron  has 
afforded  the  means  of  studying  its  developmental  stages  with 
very  satisfactory  results.  All  the  individuals,  from  the  earli- 
est observed  stage  upward,  agree  in  contour,  there  being  no 
such  variation  in  this  respect  as  has  been  noticed  in  some 
other  species  (e.  g.,  Homceospira  evax,  Whitfieldella  nitida)  in 
which  appear  deviations  from  the  normal,  producing  a  long 
type  and  a  broad  type.  The  youngest  individual  detected 
has  a  length  of  2.25  mm.  and  a  width  of  2  mm.,  though  this 
may  not  be  regarded  as  the  initial  shell  on  account  of  the 
presence  of  partially  developed  deltidial  plates.  From  this 
stage  of  growth  to  maturity,  the  material  has  afforded  every 
variation  in  size  and  structure. 

Specific  Characters. 

Mature  Form  (Plate  XX,  figures  13,  13  a,  20,  20  a).  — 
Atrypa  reticularis  is  so  widely  distributed,  historically  and 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     357 

geographically,  in  Paleozoic  faunas,  and  is  so  familiar  to 
paleontologists,  that  a  detailed  description  of  its  mature  form 
is  here  unnecessary.  It  is  sufficient  to  remark  that  the  pre- 
vailing expression  at  this  locality  does  not  precisely  conform 
to  the  type  of  A.  reticularis,  but  is  more  nearly  that  variety 
described  by  Professor  Hall  (Pal.  N.  F.,  vol.  ii,  p.  271, 
1852)  under  the  name  Atrypa  rugosa.  This  is  evident  from 
the  development  of  the  varical  lamellae,  which  over  the 
plications  are  infolded  into  nearly  tubular  processes,  some- 
times produced  at  a  strong  angle  from  the  shell  to  a  length 
of  a  millimetre  or  more.  On  the  varices  the  plications  are 
covered  by  fine  concentric  wrinkles.  The  average  size  of 
mature  individuals,  25  x  25  mm.,  is  less  than  that  usual 
to  the  species  when  occurring  in  later,  especially  Devonian, 
faunas. 

Incipient  Form  (Plate  XX,  figures  12,  12  a,  15,  15  a).  — 
The  initial  shell,  or  the  actual  inchoate  period  in  its  forma- 
tion, is  not  at  present  known.  The  incipient  shell  of  the 
series  here  studied  is  very  small,  and  can  be  but  a  few 
removes  from  the  initial  stage.  As  just  observed,  it  meas- 
ures 2. 25  mm.  in  length  by  3  mm.  in  width,  and  shows  but 
two  concentric  striae  or  growth-varices,  with  a  correspond- 
ingly slight  development  of  the  deltidial  plates,  so  that  this 
shell  may  be  regarded  as  but  two  stages  advanced  from  the 
actual  inception  of  the  shell.  The  test  is  flat,  both  valves 
being  shallow  and  depressed  toward  the  anterior  margin ;  the 
ventral  beak  high  and  erect,  the  dorsal  beak  inconspicuous 
and  rounded.  The  foramen,  which  is  undoubtedly  triangular 
in  the  initial  shell,  has,  at  this  stage,  its  basal  angles  slightly 
rounded  by  the  faintly  developed  deltidial  plates.  The  pli- 
cations are  six  in  number  on  the  ventral,  and  five  on  the 
dorsal  valve,  the  middle  one  of  the  latter  not  reaching  as  far 
toward  the  beak  as  those  adjoining  it,  and  toward  the  ante- 
rior margin  being  depressed  below  the  lateral  portions  of  the 
shell.  General  outline  sub-circular  or  sub-pentagonal,  as  in 
the  full-grown  shell. 


358  STUDIES  IN  EVOLUTION 

Developmental  Variations. 

General  Form  and  Outline.  —  Embryos  of  less  than  3  mm. 
in  length  are  more  nearly  circular  in  outline  than  at  any  sub- 
sequent period  of  the  existence  of  the  individual.  Directly 
thereafter  the  hinge -line  represents  the  greatest  diameter  of 
the  shell,  and  the  outline  becomes  sub-pentagonal,  a  feature 
which  is  more  apparent  in  young  individuals  having  between 
3  and  10  mm.  length,  as  the  increasing  rotundity  of  the  shell 
with  the  approach  of  maturity  has  a  tendency  to  obscure,  in 
a  measure,  this  outline.  At  the  earliest  stage  studied,  the 
dorsal  valve  is  distinctly  depressed  along  the  median  line, 
forming  a  sinus  containing  a  single  plication  which  does  not 
reach  to  the  beak  (Plate  XX,  figure  14,  a).  This  sinus  grad- 
ually becomes  shallower,  and  the  plications  are  increased  by 
intercalation  until  they  are  three  in  number  (figure  14,  6).  In 
the  next  stage  all  evidence  of  a  sinus  upon  the  anterior 
margin  disappears,  leaving  it  even  and  straight  as  shown  in 
figure  14,  c ;  then  the  anterior  edge  becomes  reflexed,  show- 
ing, in  subsequent  stages  of  growth,  a  fold  where  there  had 
previously  been  a  sinus,  this  fold  bearing  at  first  three,  then 
five,  and  eventually,  in  the  mature  individual,  seven  plica- 
tions (figure  14, d,  e,  /).  This  very  remarkable  reversion  of 
the  fold  and  sinus  relatively  to  the  valves  which  bear  them 
is  also  seen  in  the  species  RhyncTiotreta  cuneata,  and  in  all 
adult  specimens  may  be  clearly  traced  upon  the  earlier  or 
embryonal  portions  of  the  valves. 

Beak.  —  In  the  first  stage  the  ventral  beak  is  high  and 
slightly  resupinate,  exposing  the  foramen  in  an  inclined 
plane.  It  gradually  shortens  and  becomes  erect,  and  when 
the  shell  attains  a  length  of  8  mm.  it  is  bent  forward,  the 
cardinal  area  being  slightly  incurved.  Thereafter  the  inflec- 
tion of  the  area  increases,  concealing  first  the  deltidial  plates, 
arid  finally  the  foramen,  until  at  maturity  the  beak  lies 
appressed  upon  the  embryonal  sinus  of  the  dorsal  valve. 

Foramen.  —  In  the  initial  shell  this  is  undoubtedly  tri- 
angular and  free  from  deltidial  plates.  With  the  starting- 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     359 

point  of  the  present  series,  however,  plates  have  begun  to 
develop,  thus  narrowing  the  pedicle-aperture,  and  rounding  its 
basal  angles.  With  the  growth  of  the  plates  more  rapidly 
along  the  lower  portion  of  their  inner  edges,  the  foramen 
shortens  quickly,  while  narrowing  but  slowly,  assuming  in 
the  second  stage  (figure  16)  a  lanceolate,  in  the  third  stage 
(figure  17)  an  oval,  and  in  the  fourth  stage  (figure  18)  a 
broadly  circular  outline.  In  the  last  two  of  these  stages  the 
deltidial  plates  have  come  in  contact  with  each  other  above 
the  apex  of  the  dorsal  valve,  and  the  pedicle-aperture  itself 
has,  from  the  second,  if  not  from  the  first  stage  in  the  series, 
encroached  upon  the  apex  of  the  valve,  so  that,  as  it  attains 
a  circular  outline,  one-half  its  periphery  is  formed  by  the  sub- 
stance of  the  valve  itself,  and  the  other  half  by  the  deltidial 
plates.  From  this  stage  upward  there  is  no  apparent  change 
in  the  actual  dimensions  of  the  foramen,  and  therefore  with 
the  growth  of  the  shell  it  becomes  relatively  much  smaller. 
It  appears,  however,  that  with  the  incurving  of  the  cardinal 
area  and  the  concealment  of  the  deltidial  plates  the  foramen 
becomes  more  and  more  enclosed  by  the  apical  portion  of  the 
valve,  and  it  may  be  that  actual  contact  with  the  deltidial 
plates  in  the  last  stage  of  development  is  lost.  In  this  final 
stadium,  with  the  procumbent  position  of  the  ventral  beak 
upon  the  dorsal  valve,  the  plane  of  the  foramen  is  parallel  to 
the  surface  of  the  dorsal  valve,  and  the  aperture  is  therefore 
lost  to  sight,  or  visible  only  at  its  upper  edge. 

Plications.  —  Of  the  five  and  six  plications  visible  upon  the 
youngest  member  of  the  series,  three  or  four  appear  to  exist 
on  that  portion  of  the  shell  included  within  the  earliest 
growth-line,  that  is,  presumptively,  the  initial  shell,  and  they 
increase  by  intercalation  until,  in  the  adult,  the  average 
number  is  about  sixty  for  each  valve.  Concentric  lines  of 
growth  follow  each  other  with  unusual  rapidity,  particularly 
in  early  life. 

Summary.  —  Atrypa  reticularis,  in  the  development  of  its 
beak,  foramen,  and  deltidial  plates,  is  in  essential  harmony 
with  the  other  uniforaminate  shells  here  discussed.  The 


360  STUDIES  IN  EVOLUTION 

reversal  of  the  fold  and  sinus  is  an  interesting  but  not 
unique  feature,  and  by  the  time  it  has  been  completely 
effected  many  of  the  characters  of  maturity  have  been 
assumed.  From  the  degree  of  exposure  of  the  foramen, 
it  may  be  judged  that  the  animal  remained  attached  by  its 
pedicle  up  to  adult  growth,  but  with  full  maturity  and  the 
approach  of  senility  the  pedicle  must  have  become  atrophied 
and  the  animal  set  free. 

Homceospira  evax  Hall,  1863. 
(PLATE  XIX,  figures  1-9.) 

Retzia  evax  Hall.     Twenty-eighth   Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist,  p.  160,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  302,  1882. 

In  this  species  the  superficial  features  have  been  found  of 
much  more  permanent  character  than  is  usual  in  the  plicate 
Brachiopoda  from  this  horizon.  Not  far  from  three  thousand 
individuals  have  been  examined,  and  these  show  a  variation 
in  size  from  a  length  of  1  mm.  and  a  width  of  .8  mm.  to  a 
length  and  width  of  25  mm.  Throughout  the  younger  stages 
in  this  series  of  variations  the  feature  of  primary  importance 
in  distinguishing  the  embryo  of  this  from  those  of  other 
species,  notably  Homoeospira  sobrina,  Camarotcechia  indianen- 
sis,  and  Camarotcechia  Whitii,  is  the  sinus  which  exists  on 
both  ventral  and  dorsal  valves ;  and  of  much  accessory  value, 
the  comparatively  slight  variation  in  the  number  of  the  plica- 
tions on  the  lateral  portions  of  the  valves.  These  features 
will  be  presently  adverted  to  more  at  length. 

Specific  Characters. 

Mature  Form  (Plate  XIX,  figures  2,  2  a).  —  Shell  ovate, 
generally  longer  than  wide,  both  valves  almost  evenly  con- 
vex, and  of  about  the  same  depth.  Anterior  margin  gener- 
ally slightly  emarginate,  on  account  of  the  median  sinus 
which  exists  on  both  valves.  In  rare  instances  a  low  median 
fold  is  developed  near  the  margin  of  the  dorsal  valve. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     361 

Ventral  valve  with  the  beak  much  elevated  above  the 
dorsal,  and  incurved,  so  that  the  plane  of  the  foramen  is 
parallel  to  the  axial  plane  of  the  shell.  Foramen  circular,  or 
slightly  sub-triangular ;  deltidial  plates  generally  obscure  on 
account  of  the  infolding  of  the  beak. 

Dorsal  valve  regularly  arcuate,  except  at  the  posterior 
extremity,  where  the  beak  is  closely  incurved  beneath  the 
ventral  umbo.  The  median  sinus  usually  carries  from  three  to 
five  plications,  but  in  advanced  growth  sometimes  becomes 
filled  up  by  the  crowding  of  these  plications.  Ventral  valve 
with  a  well-marked  sinus,  generally  bearing  three  plications. 
The  sinal  plications  on  both  valves  take  their  origin  in  front 
of  the  beak,  and  are  of  interstitial  growth,  a  fact  which  does 
not  hold  true  for  any  of  the  other  plications.  The  surface  is 
marked  by  from  eight  to  twelve  rounded,  continuous  plica- 
tions on  each  side  the  sinus  of  either  valve,  all  of  these 
extending  to  the  beak,  with  the  possible  exception  of  the 
more  obscure  ones  on  the  cardinal  slopes.  Only  in  rare 
instances  and  abnormally  do  .these  plications  increase  by 
interstitial  addition.  Imbricating  lines  of  growth  are  often 
present,  and  fine  concentric  striae  are  sometimes  discernible. 

The  mature  individuals  of  Homceospira  evax  divide  them- 
selves into  three  groups,  based  on  their  relative  proportions : 

(cf)  Normal  form,  in  which  the  length  and  width  are  equal. 

(b)  Long  form,  in  which  the   length  is  greater  than  the 
width. 

(c)  Broad  form,  in  which  the  length  is  less  than  the  width. 
In  frequency  of  occurrence  the  form  (6)  almost  equals  the 

normal,  while  the  form  (<?)  is  more  rarely  met  with.  The 
form  (6)  is  also  of  remarkable  persistence,  and  starts  so  early 
in  the  life  of  the  individual  as  to  suggest  a  distinct  genetic 
impulse. 

Variations  from  the  Normal  Development.  —  These  are  to  an 
unusual  degree  very  slight,  and  may  be  classed  as  follows : 

Obesity,  which  apparently  occurs  only  when  normal  full 
growth  has  been  attained. 

A  tendency  to  asymmetry  in  the  development  of  the  siual 


362  STUDIES  IN  EVOLUTION 

plications.  A  marked  illustration  of  this  is  afforded  by  an 
individual  which,  in  repairing  an  injury  to  its  shell,  has 
abruptly  developed  six  plications  on  one  side  of  the  sinus, 
in  continuation  of  three  and  to  correspond  with  three  on 
the  opposite  side. 

The  absence  of  plications  in  the  sinus.  This  is  a  feature 
of  rare  occurrence,  and  is  undoubtedly  an  infantile  character 
retained  in  later  stages  of  growth.  A  single  individual  of 
immature  growth  affords  an  illustration  of  a  peculiar  abnor- 
mality, indicating  a  reversal  in  the  growth  to  an  embryonic 
condition.  This  shell  (Plate  XIX,  figures  3,  3  a, )  has  grown 
to  a  certain  size  and  normally  developed  its  plications,  but  an 
abrupt  period  has  been  placed  to  their  development,  and  over 
the  entire  anterior  portion  of  the  individual,  in  front  of  a 
stout  varix,  the  surface  of  the  shell  is  almost  smooth.  This 
is  the  exact  counterpart  of  that  mode  of  growth  to  which 
attention  is  called  under  other  species,  where  the  smooth 
embryonic  condition  of  the  shell  seems  to  be  prolonged  for 
more  than  the  usual  period  of  immaturity,  and  the  mature 
features  are  thereupon  abruptly  developed  after  the  formation 
of  a  sharp  growth-line. 

Developmental   Variations. 

The  series  of  individuals  illustrating  the  embryological 
changes  in  this  species  is  so  complete  as  to  show  by  almost 
imperceptible  gradations  the  entire  chain  of  development 
from  very  near  the  starting-point  up  to  maturity.  This 
series  begins  with  an  individual  measuring  1  mm.  in  length 
and  .8  mm.  in  width,  and  at  this  stage  of  growth  the  incipi- 
ent shell  has  manifestly  not  received  much  increment.  That 
this,  however,  is  not  the  actual  primitive  shell  seems  proved 
by  indications  of  two  very  indistinct  concentric  growth-lines, 
and  by  the  presence  of  faint  radiating  plications  near  the 
anterior  margin,  between  the  second  growth-line  and  the 
margin  itself.  It  is  very  probable  that  the  incipient  shell 
consisted  of  that  portion  of  the  individual  (Plate  XIX,  figure 
1)  lying  within  the  first  growth-line,  and  as  this  would  make 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     363 

its  size  about  .5  X  .4  mm.,  this  fact  in  itself  is  sufficient 
apology  for  the  writers'  not  having  detected  the  earliest  stage 
of  its  development,  even  if  other  causes  had  permitted  its 
preservation. 

Beaks.  —  In  all  normally  developed  individuals  less  than  5 
mm.  in  length  the  beak  of  the  ventral  valve  is  erect  and 
exsert.  At  about  this  stage  of  growth  a  tendency  to  apical 
incurvature  is  manifested,  which  increases  up  to  maturity, 
when,  under  normal  development,  the  entire  umbo  is  evenly 
incurved,  concealing  the  deltidial  plates  and  often  much  of 
the  foramen.  On  the  dorsal  valve  the  beak  is  quite  obscure 
in  the  youngest  forms,  and  in  later  stages  of  growth  is  con- 
cealed beneath  the  deltidium  or  incurved  beak  of  the  opposite 
valve. 

Foramen.  —  This  appears  first  as  a  simple  triangular  open- 
ing, its  apex  reaching  to,  but  not  truncating  the  apex  of  the 
umbo,  and  it  is  retained  in  this  condition  until  the  shell 
attains  a  length  of  at  least  3  mm.  of  normal  growth.  At 
this  age  the  deltidial  plates  begin  to  form,  making  their  first 
appearance  as  two  minute  triangular  laminae,  taking  their 
origin  in  the  basal  angles  of  the  foraminal  triangle,  and  giv- 
ing the  foramen  a  lanceolate  outline. 

By  increments  to  their  internal  edges,  these  plates  pres- 
ently come  in  contact  with  each  other,  truncating  the  interior 
basal  angle  of  each,  the  plates  being  from  this  period  onward 
in  progressive  symphysis.  The  increments  to  these  plates  are 
made  more  rapidly  at  and  about  their  interior  angles,  and,  as 
a  result,  the  foramen  assumes  successively  an  elliptical,  an  oval, 
and  a  circular  outline.  The  circular  curve  of  its  upper 
extremity  is  caused  by  a  slight  encroachment  upon  the  beak, 
and  this  in  mature  age  is  so  considerable  that  the  primary  or 
incipient  shell  is  undoubtedly  wholly  absorbed.  The  plane 
of  the  foramen  remains,  except  in  rare  instances,  always  ver- 
tical, although  the  deltidial  plates  become  slightly  bent  by 
the  incurving  of  the  beak.  A  striking  exception  to  this  rule 
is  represented  on  Plate  XIX,  figure  9,  where  an  individual 
which  has  reached  early  maturity  shows  the  senile  feature  of 


364  STUDIES  IN  EVOLUTION 

a  beak  incurved  to  such  a  degree  as  almost  to  obscure  the 
foramen.  On  approaching  maturity  the  deltidial  plates  ap- 
pear to  become  anchylosed  along  their  exterior  edges,  with 
the  shell  itself,  the  line  of  union  being  marked  with  a  low 
ridge,  and  seem  never  to  be  displaced  by  any  distortion  of  the 
shell,  as  so  often  occurs  in  Atrypina  disparilis. 

Sinus.  —  As  already  noticed,  in  the  elementary  shell  the 
sinus  begins  as  a  low,  smooth  depression,  equally  strong  on 
both  valves,  and  extending  almost  to  the  beak.  It  gradually 
becomes  filled  by  the  radiating  plications,  which  appear  first 
at  the  sides,  and  increase  toward  the  middle,  never  becom- 
ing, normally,  more  than  six.  None  of  these  plications 
reach  the  apex  of  the  shell. 

Plications.  —  On  the  latera  of  the  shell  these  seem  to  appear 
simultaneously,  as  shown  in  figure  1,  where  three  on  each 
side  make  their  appearance  at  the  same  stage  of  growth. 
This  number  is  subsequently  increased  to  six  or  eight  on 
each  side  in  mature  forms,  sometimes  becoming  greater  in 
old  age. 

internal  Apparatus.  —  The  brachial  supports  in  this  species 
consist  of  spirals,  coiled  in  the  transverse  axis  of  the  shell, 
with  their  bases  facing  each  other.  In  the  mature  individual 
the  number  of  coils  is  from  eight  to  ten.  The  spirals  are 
connected  by  an  angular  loop,  the  branches  of  which  take 
their  origin  on  the  dorsal  limb  'of  the  basal  coils,  and  are 
directed  ventrally  and  backward  beyond  the  axis  of  the  in- 
terior cavity,  forming  at  their  junction  not  a  simple  angle, 
but  a  miniature  saddle,  from  the  posterior  extremity  of  which 
extends  straight  backward  a  little  spiniform  process.  The 
number  of  coils  in  these  spires  varies  with  the  age  of  the 
shell.  In  preparing  a  series  to  show  the  development  of 
these  structures,  it  appears  that  the  shelly  ribbons  composing 
the  spirals  not  only  make  fewer  coils  in  early  life,  but  that 
these  are  of  exceeding  tenuity  in  the  primary  stages  of  de- 
velopment. The  accompanying  figures  show  the  extremes  of 
development  noticed  in  these  respects,  figures  126  and  127 
representing  the  character  of  the  supports  in  the  mature 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     365 

condition,  and  figure  128  the  spirals  as  developed  in  an  indi- 
vidual having  a  length  of  2.5  mm.,  where  the  ribbon  makes 
but  two  revolutions.  As  far  as  can  be  ascertained,  the  loop 
undergoes  no  essential  modification  in  these  early  stages, 
though  its  precise  character  in  the  example  from  which  this 
drawing  has  been  made  was  not  determined,  but  has  been 
drawn  in.  The  same  arrangement,  however,  has  been  seen 
in  an  individual  of  but  slightly  larger  growth. 

The  growth  of  these  spirals  consists,  primarily,  in  the  addi- 
tion to  the  number  of  coils,  and,  secondarily,  in  the  thicken- 


126 


127 


128 


FIGURES  126-128.  —  Development  of  internal  apparatus  in  Homceospira  evax 
Hall. 


ing  of  the  ribbon.  In  the  first  case  the  increase  in  number 
must  take  place  by  addition  to  the  apices  of  the  coils,  and 
therefore  the  embryonic  or  primary  coils  of  the  ribbon  must 
be  wholly  concealed  by  later  depositions  upon  them,  both  in 
length  and  width.  The  apparent  looseness  of  the  coils  in 
their  primary  condition,  however,  must  be  regarded  as  largely 
due  to  magnification,  the  distance  between  successive  coils 
being  actually  not  so  great  as  the  distance  between  the  apical 
turns  of  the  ribbon  in  the  mature  spiral. 


366  STUDIES  IN  EVOLUTION 

Homceospira  sobrina  sp.  nov.* 

(PLATE  XIX,  figures  10-16.) 
Rhynchonella  Whitii  Hall,  in  part. 

In  the  examination  of  a  large  number  of  the  specimens 
which  have  usually  passed  under  the  name  of  Camarotoechia 
Whitii,  the  writers  have  become  convinced  that,  aside  from  the 
individuals  which  agree  with  the  types  and  the  description  of 
the  species,  there  is  a  series  of  shells  which,  in  the  mature 
state,  may  be  readily  confounded  with  immature  stages  of  0. 
Whitii,  but  in  their  immature  condition  are  readily  separable 
from  this  species,  and  form,  of  themselves,  a  satisfactory  and 
well-defined  developmental  series.  The  similarity  of  these 
examples  with  0,  Whitii  is  found  in  the  general  outline,  the 
strong,  simple  plications  approximately  the  same  in  number, 
and  the  usual  two  plications  on  the  median  fold.  The  exter- 
nal differences,  however,  in  the  new  species  are  these:  The 
plications  on  the  fold  may  be  one  or  three,  and  whatever 
their  number,  the  fold  is  always  depressed,  in  most  instances 
even  to  obsolescence,  and  the  plications  upon  it  are  low  and 
often  faint.  The  foramen,  also,  is  circular  in  maturity,  with 
perfectly  developed  deltidial  plates,  and  the  surface  of  the 
valves  usually  conspicuously  marked  by  fine,  crowded,  con- 
centric growth-lines.  Internally  the  difference  is  more  em- 
phatic, as  carefully  prepared  specimens  show  well-defined 
spirals  having  their  apices  near  the  lateral  margins,  as  shown 
on  Plate  XIX,  figure  12.  While  disavowing  the  intention  of 
describing  new  species  as  remote  from  the  purposes  of  this 
paper,  the  writers  have,  for  convenience  in  utilizing  this  form 
for  their  work,  to  which  it  makes  no  unimportant  contribution, 
designated  it  as  above,  as  no  doubt  exists  of  its  specific  value. 

Camarotcechia  sobrina,  one  of  the  more  abundant  of  the 
Waldron  Brachiopoda,  is  itself  subject  to  some  variation, 
more  considerable,  indeed,  than  that  noticed  in  either  of  the 
species  Camarotcechia  Whitii  and  C.  neglecta.  The  material 

[*  Originally  referred  to  the  genus  Retzia.~\ 


DEVELOPMENT  OF  SOME   SILURIAN  BRACHIOPODA     367 

represents  all  developmental  stages  between  the  limits  of 
these  dimensions:  2  X  1.6  mm.  (incipient  shell)  and  7  x  6.5 
mm.  (maturity).  In  its  youngest  stages  it  shows  a  certain 
degree  of  similarity  with  Homoeospira  evax^  especially  in  the 
sinus  on  both  valves  and  in  the  sinal  plications.  The 
greater  number  of  the  latter  in  H.  evax,  as  well  as  the  more 
numerous  lateral  plications,  will  serve  to  obviate  confusion 
here. 

Specific  Characters. 

Mature  Form  (Plate  XIX,  figures  11,  11  a,  11  £).  —  Shell 
small,  rotund,  in  outline  broadly  ovate  to  sub-pentagonal. 
Valves  of  equal  convexity. 

Ventral  valve  with  umbo  prominent,  attenuate,  erect,  and 
slightly  incurved  at  the  apex;  cardinal  margins  not  excavate, 
sloping  with  a  faint  curve  to  the  sides,  whence  they  round 
to  the  anterior  edge,  which,  in  the  sinal  region,  is  nearly 
straight;  cardinal  area  distinct;  foramen  circular;  deltidial 
plates  prominent. 

Dorsal  valve  sub-circular  in  outline,  arched  in  the  umbonal 
region;  beak  well-defined,  apex  concealed.  In  the  umbonal 
region  the  median  portions  of  both  valves  are  slightly  more 
convex  than  elsewhere,  but  this  prominence  disappears  toward 
the  margins,  the  valves  becoming  slightly  flattened  and  de- 
pressed on  the  median  region  near  the  anterior  margin,  mak- 
ing a  low  sinus  on  the  ventral,  and  a  low,  depressed  fold  on 
the  dorsal  valve.  Both  fold  and  sinus  may  bear  one,  two, 
or  three  small,  often  faint  and  unsymmetrically  developed 
plications,  the  strongest  of  which  may  have  its  origin  in  the 
umbonal  region,  while  the  others  rarely  extend  more  than 
half-way  across  the  shell.  On  each  of  the  latera  are  four 
or  five  strong,  angular,  simple  plications,  making  thus  from 
nine  to  thirteen  plications  on  each  valve.  The  increase  in 
these  takes  place  altogether  on  the  fold  and  sinus,  the  full 
quota  of  lateral  plications  appearing  early  in  the  history  of 
the  individual.  The  plications  are  covered  by  numerous 
fine,  concentric  growth-lines  more  noticeably  developed  near 
the  margin,  and  at  intervals  becoming  varicose. 


368  STUDIES  IN  EVOLUTION 

The  spiral  brachial  supports  each  make  about  five  revolu- 
tions, which  come  to  an  apex  near  the  lateral  margins. 

Variations  from  the  Normal  Mature  Form.  —  An  elongate 
form  with  an  unusually  high  and  straight  beak  is  not  of  rare 
occurrence,  and  is  the  resultant  of  a  very  completely  repre- 
sented series  of  embryonic  stages.  The  species  is  also  sub- 
ject to  obese  growth,  resulting  from  two  sources :  (a)  general 
internal  thickening  of  the  shell,  (b)  marginal  thickening. 
Both  are  the  result  of  post-adolescent  or  senile  growth,  the 
former  producing  a  round,  full,  plethoric  shell,  the  latter 
giving  the  shell  a  truncate  appearance. 

incipient  Form  (Plate  XIX,  figures  10,  10 a).  —The  first 
stage  of  growth  represented  in  this  series  measures  2  mm.  in 
length  by  1.6  mm.  in  width.  The  valves  are  sub-equally 
convex,  somewhat  depressed  anteriorly.  Outline  broadly 
ovate. 

Ventral  valve  with  beak  high,  erect,  and  sub-acute;  car- 
dinal slopes  broad,  steep,  and  slightly  excavate;  pedicle- 
aperture  sub-triangular,  rounded  at  the  apical  angle,  and  also 
slightly  at  the  base,  by  the  already  developed  deltidial  plates. 

Dorsal  valve  sub-circular,  beak  full,  rounded,  sides  slightly 
appressed,  apex  concealed.  Surface  of  the  dorsal  valve 
marked  by  two  thread-like  plications  which  take  their  origin 
medially,  just  below  the  umbonal  region ;  thenceforward  they 
rapidly  diverge,  forming  the  embryonal  sinus,  which  is,  how- 
ever, soon  filled  by  two  small  plications.  The  latera  each 
bear  one  plication,  of  earlier  age  than  the  sinal,  and  later 
than  the  primary  plications.  On  the  ventral  valve  the  plica- 
tions number  the  same,  but  the  embryonal  fold  or  dorsum  is 
more  strongly  marked  than  the  dorsal  sinus.  On  both  valves 
indications  of  the  mature  fold  and  sinus  begin  with  the  appear- 
ance of  the  sinal  plications. 

Developmental  Variations. 

General  Form  and  Outline*.  —  There  is  a  gradual  increase  in 
convexity  and  diameter  with  each  successive  growth-stage, 
until  maturity  is  reached.  The  elongate  variation  from  the 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     369 

normal  seems  to  have  retained  through  all  stages  of  growth 
the  proportions  of  the  normal  embryo. 

Beak  and  Foramen.  —  The  erect,  straight,  sub-acute  beak 
of  the  incipient  shell  in  later  growth  becomes  rounded  and 
slightly  arched  or  incurved.  The  cardinal  area  in  all  stages 
of  development  is,  however,  high,  exceptions  being  made  for 
the  more  extreme  cases  of  obesity,  where  the  deltidial  plates 
may  be  concealed,  but  the  foramen  is  always  exposed,  and 
the  beak  is  never  procumbent  on  the  opposite  umbo.  The 
plates  arising  from  the  base  of  the  thickened  foraminal  mar- 
gins meet  in  such  a  manner  as  to  leave  the  foramen  sharply 
acute  below,  and  sub-triangular.  By  their  subsequent  up- 
ward growth  arid  more  complete  union  the  foramen  becomes 
circular,  the  lines  of  symphysis  with  the  valves  still  remain- 
ing thickened. 

In  occasional  instances  a  probable  slight  displacement  of 
the  plates  outwardly  along  the  median  suture  gives  them  the 
appearance  of  sloping  away  from  the  median  to  the  lateral 
sutures. 

Atrypina  disparilis  Hall,  1852. 
(PLATE  XIX,  figures  17-23.) 

Ccelospira  disparilis  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus. 

Nat.  Hist,,  p.  162,  pi.  25,  figs.  39-43,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  303,  pi.  25, 

figs.  39-43,  1882. 

Although  this  species  is  one  of  the  less  abundant  members 
of  the  Waldron  fauna,  several  hundred  immature  individuals 
have  been  found,  the  youngest  of  which  has  dimensions  of 
2.5  mm.  in  length  by  2  mm.  in  width,  the  greatest  size  at  ma- 
turity being  6.5  mm.  in  length  by  6  mm.  in  width.  The  species 
being  in  its  mature  size  quite  small  and  in  its  surface  feat- 
ures quite  simple,  it  does  not  afford  such  scope  for  variations 
through  the  later  embryonic  stages  as  many  of  its  associated 
species,  and  hence  it  will  be  noticed  that  in  surface  sculpture 
a  permanency  of  character  is  retained  through  all  stages  of 
growth. 

24 


370  STUDIES  IN  EVOLUTION 

Specific  Characters. 

Mature  Form  (Plate  XIX,  figures  19,  19  a,  195).— Shell 
small,  sub-oval  or  cordate,  often  sub-pentagonal;  plano- 
convex, greatest  width  along  the  hinge. 

Ventral  valve  convex,  depressed  at  the  sides ;  beak  exsert, 
and  in  old  forms  arcuate. 

Dorsal  valve  flat  and  sometimes  depressed  near  the  beak. 
The  ventral  valve  is  marked  by  two  very  prominent  plica- 
tions which  pass  along  the  deep  median  sinus,  and  are  accom- 
panied by  two  less  distinct  plications  on  each  of  the  latera, 
making  in  all  six  plications,  of  which  the  two  nearest  the 
hinge -line  are  sometimes  obsolescent.  The  dorsal  valve  bears 
a  prominent  median  fold,  and  two  well-marked  plications  on 
the  lateral  portions  of  the  shell.  Toward  the  margin  are 
concentric  laminae  of  growth.  An  average  adult  individual 
measures  5.5  mm.  in  length  by  5  mm.  in  width. 

Variations  in  Outline.  —  The  individuals  divide  themselves 
into  two  groups  according  to  their  outline : 

(a)  Normal  form. 

(b)  Long  form. 

The  first  of  these  groups  includes  the  great  majority  of  all 
individuals  found,  which  are  characterized  by  a  relatively 
broad  figure  and  sub-circular  outline.  Members  of  the 
second  group  are  comparatively  few  in  number,  and  are 
elongate  or  sub-ovate  individuals.  The  long  form  (b)  is  well 
defined  in  immature  growth-stages,  and  appears  to  be  a 
permanent  varietal  difference. 

Abnormalities.  —  A  variation  in  adult  shells,  noticed  only 
in  rare  instances,  is  a  tendency  to  an  asymmetrical  develop- 
ment in  the  plications,  as  shown  on  Plate  XIX,  figure  18,  where, 
by  unequal  growth  upon  the  lateral  portions  of  the  shell,  the 
median  plication  on  the  dorsal  valve  is  deflected  to  one  side, 
and  the  corresponding  median  sinus  on  the  ventral  valve  dis- 
placed, the  axial  line  of  the  shell  being  occupied  by  one  of 
the  strong  plications  bounding  the  sinus.  Another  form  of 
this  asymmetry  is  manifested  in  the  intercalary  addition  of  a 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     371 

single  plication  on  one  side  of  the  median  plication  of  the 
dorsal  valve. 

A  tendency  to  obesity  is  often  manifested  by  the  shell,  at 
or  before  reaching  the  average  dimensions  of  maturity,  when 
it  may  be  supposed  that  the  full  growth  of  the  individual  has 
been  attained.  This  obesity  is  produced  by  a  rapid  thicken- 
ing of  the  shell  at  the  margins,  making  the  anterior  face 
truncate  and  forcing  the  ventral  beak  over  the  dorsal  in  the 
same  manner  as  if  the  valves  were  forced  to  open  along 
the  hinge.  It  is,  therefore,  only  in  individuals  which  have 
reached  this  obese  condition  that  the  ventral  beak  is  incurved. 

Developmental  Changes. 

The  character  of  the  primal  or  elemental  shell  may  be  seen 
from  a  single  example  (Plate  XIX,  figure  18),  in  which  the 
plications  are  abruptly  developed  at  a  distance  of  1.5  mm. 
from  the  apex,  and,  presumably,  that  portion  of  the  shell 
within  this  limit  represents  approximately  the  size  of  the 
original  embryonic  shell.  This1  portion  of  the  individual  is 
quite  smooth,  and  shows  but  a  trace  of  the  median  fold  and 
sinus.  As  already  observed,  there  is  a  marked  permanency 
in  the  surface  features  of  the  species  from  early  youth  to 
maturity.  The  smallest  individual  obtainable  bears,  as  in 
the  mature  condition,  six  plications  on  the  ventral  and  five 
on  the  dorsal  valve,  though  those  near  the  hinge-line  are  quite 
faint. 

The  beak  is  prominent  and  exsert,  except  in  obese  shells, 
where  it  is  incurved.  In  the  earliest  stage  where  the  char- 
acter of  the  foramen  is  well  preserved  the  individual  has  a 
length  of  4  mm.  and  a  width  of  3.5  mm.  Here  it  is  seen  to 
be  elongate-oval,  the  deltidial  plates  having  formed  to  such 
a  degree  as  to  be  in  contact  with  each  other  and  to  have 
anchylosed,  so  that  the  median  suture  is  detected  with  diffi- 
culty. The  lateral  sutures  always  remain  distinct  even  to 
maturity,  and  it  is  evident  that  the  union  of  the  plates  with 
the  shell  along  these  joints  has  not  been  as  firm  as  in  many 
species,  as  it  is  not  infrequently  found  that  the  plates  have 


372  STUDIES  IN  EVOLUTION 

been  displaced  and  lost.  Careful  search  among  the  smallest 
individuals  has  shown  no  trace  of  the  inceptive  triangular 
outline  of  the  pedicle-groove  existing  in  other  species  before 
the  formation  of  the  deltidial  plates.  It  is  an  important  fact 
that  the  foramen  begins  to  assume  its  mature  condition  so 
early  in  the  history  of  the  shell,  although  its  development 
was  evidently  in  conformity  with  the  general  type. 

The  subsequent  development  of  the  deltidial  plates  changes 
the  form  of  the  foramen  to  that  of  a  circle,  as  shown  in  figures 
22  and  23.  In  the  early  life  of  the  shell  the  plane  of  the 
foramen  is  in,  or  parallel  to,  the  axial  plane;  at  maturity, 
before  any  obesity  or  senile  thickening  takes  place,  the  fora- 
men, in  becoming  less  elongate,  truncates  the  apex  of  the 
valve,  and  makes  a  large  angle  (sometimes  almost  90°)  with 
the  axial  plane;  subsequently,  with  increase  in  obesity,  it 
becomes  again  more  nearly  parallel  to  this  plane.  In  the 
last  condition  the  deltidial  plates  are  curved  inward,  and 
often  to  a  large  degree  concealed. 

Meristina  rectirostris  Hall,  1882. 

(PLATE  XXI,  figures  4,  5,  11-13.) 

Meristella  rectirostra  Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana, 
p.  301,  pi.  27,  figs.  10-14,  1882. 

This  small  species  is  one  of  the  less  abundant  of  the 
brachiopods  of  this  fauna,  and  probably  has  often  been  con- 
founded with  undeveloped  individuals  of  Whitfieldella  nitida. 
It  presents,  however,  adult  features  which  will  not  allow  it 
to  be  confounded  with  that  species,  and  although  some  diffi- 
culty arises  in  separating  the  diminutive  forms  of  the  two 
species,  M.  rectirostris  is  characterized  by  the  absence  of 
deltidial  plates  in  every  stage  of  its  existence. 

The  series  representing  this  species  does  not  include  stages 
of  growth  as  early  as  in  some  of  the  others,  but  is  sufficiently 
complete  to  permit  the  statement  that,  were  younger  forms 
accessible,  they  would  probably  add  little  to  a  knowledge 
of  the  developmental  changes.  The  series  begins  with  indi- 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     373 

viduals  having  a  length  of  2.5  mm.  and  a  width  of  1.75  mm., 
the  adult  form  measuring  9  mm.  in  length  by  7.5  in  width. 

In  all  stages  of  growth  earlier  than  that  approximately 
indicated  by  a  size  of  6  X  4  mm.,  it  is  very  difficult,  and  from 
the  present  observations  impossible,  to  draw  the  line  of  separa- 
tion between  this  species  and  W.  nitida,  and  the  fact  which 
has  been  demonstrated  for  Dalmanella  elegantula  and  Rhipid- 
omella  hybrida  ;  namely,  that  in  the  earliest  growth-stages  no 
specific  differences  are  manifest,  will  be  probably  found  to 
hold  good  for  these  two  species  also.  And  in  the  latter  case 
a  considerably  larger  size  is  attained  by  the  embryonic  forms 
than  is  reached  by  the  former  species,  before  the  differential 
characters  are  assumed.  This  is  due  to  the  fact  that  these 
two  species,  when  mature,  have  essentially  no  surface  sculp- 
ture, and  differ  less  in  general  form  and  outline  than  do  the 
mature  individuals  of  Dalmanella  and  Rhipidomella. 


Specific  Characters. 

Mature  Form  (Plate  XXI,  figures  12,  12  a,  12  5).  —  Shell 
sub-pentagonal  or  ovate;  beak  erect,  acute,  and  prominent, 
rapidly  widening  toward  the  base.  Lateral  margins  nearly 
straight  for  about  one-third  the  length  of  the  shell,  thence 
rounding  to  the  anterior  margin.  Valves  about  equally  con- 
vex, giving  the  shell  a  sub-lenticular  contour. 

Ventral  valve  with  attenuate,  straight,  or  slightly  arcuate 
beak.  Foramen  triangular  and  without  deltidial  plates. 

Dorsal  valve  more  nearly  sub-pentagonal  in  outline ;  beak 
incurved  into  the  foramen  of  the  ventral  valve. 

Surface  smooth,  or  in  rare  instances  showing  a  faint 
pseudo-punctate  appearance  which  is  entirely  superficial. 
Dimensions  of  average  adult  9  X  7.5  mm. 

Incipient  Form  (Plate  XXI,  figures  4,  4  a).  —  Shell  measur- 
ing 2.5  x  1.75  mm.  Oval,  proportionally  longer  and  nar- 
rower than  in  the  adult  state.  Beak  elevated,  acute,  straight. 
Foramen  of  the  ventral  valve  very  broad,  triangular,  extending 
to  the  apex.  Dorsal  beak  full,  rounded,  and  inconspicuous. 


374  STUDIES  IN  EVOLUTION 

Shell  convex  just  below  the  beak,  becoming  depressed  toward 
the  anterior  margin. 

Developmental  Variations. 

General  Form  and  Outline.  —  In  the  incipient  stadia  of 
growth  the  shell  is  extremely  elongate  and  quite  perfectly 
oval;  the  beak  of  the  ventral  valve  is  relatively  broad,  its 
lateral  margins  having  a  slight  outward  curve.  With  growth 
the  shell  broadens,  and  the  ventral  beak  becomes  more  atten- 
uate, while  the  greatest  width  of  the  shell,  instead  of  being 
at  or  below  the  middle,  comes  nearer  the  hinge-line. 

Beak.  —  From  being  erect,  straight,  and  relatively  broad 
in  the  ventral  valve,  at  the  outset,  it  becomes,  at  maturity, 
narrow,  attenuate,  and  slightly  incurved  toward  the  apex. 

Foramen.  —  In  the  earliest  observed  stage  the  foramen  is 
a  broad,  triangular  opening,  covering  nearly  the  entire  car- 
dinal area,  reaching,  but  not  encroaching  upon,  the  apex  of 
the  valve.  In  subsequent  stages  of  development  this  open- 
ing narrows  with  the  narrowing  of  the  beak  but,  as  at  no 
stage  deltidial  plates  are  developed,  the  contraction  is  due  to 
the  encroachment  of  the  cardinal  portions  of  the  valve  along 
the  foraminal  margins.  The  interesting  fact  of  the  persistent 
absence  of  deltidial  plates  throughout  the  entire  existence  of 
the  individual  may  be  interpreted  as  a  retention  to  maturity 
of  a  character  embryonic  in  allied  species ;  the  small  size  of 
the  mature  shell  and  the  very  slight  incurvature  of  the  ven- 
tral beak  also  contribute  to  the  embryonic  expression  of  the 
species. 

Whitfieldella  nitida  Hall,  1843. 

(PLATE  XXI,  figures  6-10.) 

Meristina  nitida  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  160,  pi.  25,  figs.  1-7,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  300,  pi.  25, 

figs.  1-7,  1882. 

Whitfieldella  nitida  is  a  very  abundant  and  characteristic 
fossil  in  the  Niagara  fauna  of  central  Indiana,  reaching  a 
much  greater  development  both  in  size  and  numbers  than  in 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     375 

the  New  York  outcrops  of  the  formation.  The  individuals 
vary  in  size  from  a  length  of  2.5  mm.  and  a  width  of  1.75 
mm.,  to  a  length  of  25  mm.  and  a  width  of  22  mm.,  and  the 
series  representing  these  variations  is,  on  account  of  the 
abundance  of  specimens,  very  complete  within  these  limits. 
It  is,  however,  a  noticeable  feature  of  the  species  that  in 
most  respects,  except  size,  the  characters  of  maturity  are 
assumed  early  in  the  life  of  the  individual,  and  as  the  form 
is  essentially  devoid  of  surface  sculpture,  the  interest  in 
its  development  rests  to  a  larger  degree  than  usual  upon 
abnormalities  in  individuals  either  mature  or  approaching 
maturity. 

Specific  Characters. 

Mature  Form  (Plate  XXI,  figures  9,  9  a,  9  5).  —  Shell  broadly 
sub-pentagonal  to  ovoid;  beaks  extended  and  more  or  less 
prominent. 

Ventral  valve  with  the  greatest  convexity  at  about  one- 
third  the  length  of  the  shell  in  front  of  the  beak.  Beak 
arched,  incurved  over  the  dorsal  valve;  apex  evenly  trun- 
cated, the  circular  foramen  lying  in  a  vertical  plane.  Cardi- 
nal slopes  extending  for  more  than  one-half  the  length  of  the 
shell.  A  low  median  depression  is  noticeable  on  the  younger 
portions  of  the  valve. 

Dorsal  valve  with  a  similar  convexity ;  beak  incurved  and 
concealed.  A  very  low  and  inconspicuous  median  elevation 
corresponds  with  the  depression  on  the  opposite  valve. 

Surface  smooth  or  with  fine  concentric  strise  and  a  few 
conspicuous  lines  of  growth  toward  the  anterior  margin. 
Average  individuals  measure  20  x  16  mm.,  large  examples 
not  infrequently  25  X  21  mm. 

Variations  in  Outline.  —  Two  very  distinct  groups  of  forms 
are  evident  in  this  species,  in  one  of  which,  (a)  normal,  the 
outline  of  the  mature  shell  is  obcordate  or  sub-pentagonal. 
When  immature,  the  anterior  margin  is  evenly  circular,  but 
in  all  cases  the  proportion  of  length  to  width  is  essentially 
the  same.  Probably  five-eighths  of  the  specimens  found 
belong  to  this  group. 


376  STUDIES  IN  EVOLUTION 

In  a  second  group,  (6)  long  form,  the  shell  is  elongate- 
spatulate,  and  proportionately  deeper  than  the  normal,  but, 
with  a  single  exception,  individuals  have  not  been  observed 
to  exceed  a  length  of  10  mm.  and  a  width  of  7  mm.  This 
variation  is  so  persistent  that  it  appears  to  be  well  founded 
genetically,  and  not  merely  an  occasional  occurrence.  Trac- 
ing backward  from  the  mature  shell  to  the  earlier  stages  of 
development,  both  this  and  the  normal  form  are  found  merging 
into  each  other ;  hence  both  have  had  a  similar  starting-point. 
The  long  form,  however,  reaches  maturity  of  development  at 
a  very  early  age,  and  never  approaches  the  size  or  proportions 
of  the  normal  adult.  A  tendency  to  obesity  is  especially 
noticeable  in  the  group  (I ),  the  majority  of  such  individuals 
being  below  the  normal  full-growth. 

A  single  adult  example  shows  traces  of  broad,  rounded 
plications  on  each  side  of  the  fold  and  sinus,  a  singular 
condition  in  a  species  uniformly  non-plicate. 

Incipient  Form  (Plate  XXI,  figures  4,  4  a,  10, 10  a).  —  Shell 
2.5  mm.  in  length  by  1.75  mm.  in  width;  elongate-oval; 
beak  elevated,  straight,  acute.  Pedicle-aperture  of  the  ven- 
tral valve  very  broad,  triangular,  extending  to,  but  not 
encroaching  upon  the  apex.  Dorsal  beak  full,  rounded,  but 
inconspicuous.  Valves  convex  just  below  the  beaks,  becom- 
ing depressed  toward  the  anterior  margin.  The  shell  is 
proportionally  much  narrower  than  the  adult  form. 

The  starting-point  of  this  series  is  precisely  the  same  form 
of  shell  as  that  taken  for  the  incipient  stage  in  the  species 
Meristina  rectirostris.  Under  the  discussion  of  that  species 
reference  has  been  made  to  the  impossibility  of  separating 
these  two  species,  in  their  earlier  stages,  and  the  impression 
of  the  specific  characters  may  be  regarded  as  of  subsequent 
development. 

Developmental  Variations. 

The  surface  characters  being  unvariable,  the  important 
changes  in  development  are  confined,  as  far  as  observable,  to 
the  pedicle-aperture  and  deltidial  plates.  As  already  observed, 
the  beak  is  incurved  so  early  in  the  history  of  the  individual 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     377 

that  these  embryological  changes  can  be  observed  only  in  very 
young  specimens.  This  incurvature  of  the  ventral  beak 
appears  to  become  fixed  earlier  in  the  normal  than  in  the 
elongate  form.  For  example,  figure  10,  Plate  XXI,  represents 
an  elongate  individual  with  a  length  of  2.5  mm.  and  a  width 
of  1.5  mm.,  with  an  open  triangular  foramen,  and  no  appar- 
ent development  of  the  deltidial  plates ;  but  the  normal  form 
of  the  same  size  has  the  plates  developed,  the  foramen  nearly 
circular,  and  the  beak  incurved.  In  the  condition  repre- 
sented in  this  figure,  the  embryos  of  this  species  are  readily 
confounded  with  Meristina  rectirostris,  in  which  the  triangu- 
lar aperture  is  retained  until  maturity.  The  latter  species  is, 
however,  distinguishable  in  all  the  later  stages  of  its  existence 
by  the  body  of  the  shell  being  broader  and  the  ventral  beak 
narrower  and  more  attenuate. 

Individuals  which  show  the  deltaria  in  their  different 
phases  are  difficult  to  obtain  on  account  of  the  tendency  of 
the  beak  to  incurvature  as  soon  as  the  plates  begin  to  form. 
An  individual  is  represented  in  figure  7,  Plate  XXI,  of  some- 
what abnormal  height  of  beak,  showing  an  intermediate  stage 
of  growth  in  the  plates  and  the  formation  of  the  foramen; 
and  in  figure  8  an  individual  of  the  same  size,  with  the 
foramen  circular  and  the  deltaria  completed  and  concealed 
by  the  infolding  of  the  beak. 

Meristina  Maria  Hall,  1863. 

(PLATE  XXI,  figures  1-3.) 

Meristella  Maria  Hall.     Trans.  Albany  Inst.,  vol.  iv,  p.  212,  1863. 
Meristina  Maria  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  159,  pi.  25,  figs.  8-12,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  299,  pi.  25, 

figs.  8-12,  1882. 
Wldtfieldia  tumida*  (Dalman  sp.)  Davidson.     Supp.  Brit.  Sil.  Brach., 

p.  107,  1882. 

This  species  may  be  regarded  as  presenting  a  general 
external  form  and  effect  diametrically  opposed  to  that  in 

*  The  late  Mr.  Davidson  identified  the  Waldron  species  with  the  Atrypa 
tumida  of  Dalinan,  the  type  of  his  genus  Whitfieldia. 


3T8  STUDIES  IN  EVOLUTION 

Meristina  rectirostris.  To  the  erect,  attenuate,  acute  beak, 
open  pedicle-aperture,  shallow  valves,  and  asinuate  anterior 
margin  of  the  latter,  the  full,  rounded,  incurved  beak,  con- 
cealed cardinal  area,  ventricose  valves,  and  strongly  sinuate 
anterior  margin  of  M,  Maria  are  strongly  contrasted.  Be- 
tween the  mature  characters  of  these  two  species,  Whitfieldella 
nitida  is  conspicuously  mediate. 

Immature  specimens  of  M.  Maria  are  far  from  abundant. 
Indeed,  the  present  series  shows  only  about  thirteen  different 
grades  of  development,  and  the  smallest  individual  which 
can  be  referred  with  certainty  to  the  species  measures  6x6 
mm.  (adult  29  mm.  in  length  by  32  mm.  in  width). 

The  writers  have,  however,  assigned  to  the  species  a  minute 
embryo  measuring  .75  X  .75  mm.,  and  if  this  is  correctly  done, 
the  embryos  of  this  species  in  the  earliest  stages  of  growth 
differ  from  those  of  the  other  non-plicate  species  here  dis- 
cussed, in  a  much  stronger  tendency  toward  a  circular  outline. 

The  beak  of  the  ventral  valve  becomes  incurved,  and  the 
cardinal  area  obscured  very  early,  so  that  the  discussion  of 
the  development  of  these  parts  is  necessarily  much  curtailed. 

Specific  Characters. 

Mature  Form  (Plate  XXI,  figures  3,  3  a).  —  Shell  compara- 
tively large,  ventricose,  broadly  ovate  or  sub-pentagonal. 

Ventral  valve  gibbous  in  the  umbonal  region,  with  a  low, 
broad  dorsum  extending  from  the  umbo  to  near  the  middle 
of  the  valve,  where  it  becomes  flattened,  sinuate,  and  at  the 
anterior  margin  is  reflected  dorsally  into  a  linguiform  exten- 
sion. Beak  closely  incurved  over  the  dorsal  valve,  fully 
concealing  the  foramen.  Cardinal  slopes  angulate  and 
slightly  excavate. 

Dorsal  valve  evenly  convex,  somewhat  gibbous,  strongly 
arcuate  transversely  along  the  dorsum,  which  becomes  ele- 
vated into  a  low  fold,  deeply  emarginate  in  front  for  the 
reception  of  the  extension  from  the  opposite  valve.  Beak 
obtuse,  incurved,  and  concealed. 

Surface  smooth,  marked  by  concentric  growth-lines  near 
the  margin. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     379 

Occasionally  individuals  of  large  growth  show  a  greater 
length  than  breadth,  presenting  an  elongate  form,  but  this 
variation  seems  to  be  due  to  more  rapid  axial  growth  after 
the  attainment  of  adult  size,  and  does  not  manifest  itself  in 
the  incompletely  developed  shells. 

Incipient  Form  (Plate  XXI,  figures  1,  1  a,  2,  2  a).  —  The 
minute  shell  which  appears  to  have  been  the  initial  form  for 
the  species  has  a  circular  outline  and  depressed  convex 
valves.  The  ventral  valve  is  evenly  convex,  with  the  beak 
erect,  short,  and  broad.  The  cardinal  area  is  low,  the  foram- 
inal  aperture  triangular,  reaching  to,  but  not  encroaching 
upon  the  beak.  The  deltidial  plates  are  absent.  Dorsal 
valve  with  the  beak  not  incurved,  but  inconspicuous.  Neither 
valve  bears  any  trace  of  a  median  elevation  or  depression. 

Developmental  Variations. 

General  Form  and  Outline.  — An  inclination  toward  a  lentic- 
ular form  and  circular  outline  is  noticeable  in  all  immature 
individuals.  Until  a  size  of  "about  18  x  18  mm.  is  attained, 
there  is  rarely,  if  ever,  any  trace  of  the  strong  marginal  fold 
of  maturity. 

Beak.  —  The  low  but  erect  ventral  beak  of  the  initial  shell 
has,  in  the  next  stage  of  growth,  become  inflected  and  obtuse, 
not,  however,  so  as  to  conceal  the  foramen,  which  remains 
apparent  above  the  apex  of  the  dorsal  valve,  until  the  rapid 
increase  in  convexity,  which  immediately  precedes  maturity, 
sets  in.  Thereafter  the  ventral  beak  becomes  more  closely 
incurved,  and  thrust  over  upon  the  dorsal  valve,  to  the  loss 
of  all  external  trace  of  the  cardinal  area. 

Foramen.  —  The  elemental  hiatus  is  shown  in  the  initial  shell, 
and  the  subsequently  developed  deltidial  plates  appear  in  the 
next  growth-stage.  In  the  latter  case  the  foramen  has 
become  nearly  if  not  quite  enclosed,  and  has  also  encroached 
upon  the  apical  portion  of  the  valve,  which  forms  about  one- 
half  its  periphery.  In  all  subsequent  stages  of  growth  the 
deltidial  plates  are  concealed,  and  whatever  portion  of  the 
foramen  appears  thereafter  above  the  dorsal  valve  is  enclosed 


380  STUDIES  IN  EVOLUTION 

by  the  circumbonal  tract.  With  the  approach  of  maturity 
this  gradually  disappears,  and  at  full  growth  every  trace  of 
it  has  become  obliterated. 


Spirifer  crispus  Hisinger,  1826. 

(PLATE  XX,  figures  6,  7.) 

Spirifer  crispus,  var.  simplex  Hall,  1879. 
(PLATE  XX,  figures  4,  5.) 

Reticularia  bicostata  Vanuxem,  1842,  var.  petila 
Hall,  1879. 

(PLATE  XX,  figures  1-3.) 

Spirifera  crispa  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  157,  pi.  24,  figs.  6-12,  19,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  295,  pi.  24. 

figs.  6-12,  19,  1882. 
Spirifera  crispa,  var.  simplex   Hall.     Twenty-eighth  Ann.  Kept.  N.  Y. 

State  Mus.  Nat.  Hist.,  p.  157,  pi.  24,  figs.  1-5,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  286, 

pi.  24,  figs.  1-5,  1882. 
Spirifera  bicostata  ?  var.  petila  Hall.     Trans.  Alb.  Inst.,  vol.  x,  abstract, 

p.  15,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  297, 

pi.  27,  figs.  8,  9,  1882. 

The  three  forms  which  are  here  treated  together  are  closely 
allied  in  all  their  general  characters.  It  is  in  their  initial 
stages,  however,  that  the  resemblance  becomes  more  than 
superficial,  for,  in  young  shells  of  less  than  2  mm.  in  length, 
it  is  difficult,  and  sometimes  impossible,  to  refer  them  to  any 
one  of  the  three  groups.  A  general  expression  of  the  com- 
mon characters  is  furnished  by  the  young  of  Spirifer  crispus, 
var.  simplex,  illustrated  by  figure  4,  on  Plate  XX. 

Taking  Reticularia  bicostata,  var.  petila,  as  the  simplest 
form,  the  young  shell  is  found  to  be  nearly  circular  in  outline, 
with  a  single,  broad,  median  fold  on  the  dorsal  valve.  Pass- 
ing to  £  crispus,  var.  simplex,  of  the  same  size,  the  outline 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     381 

is  seen  to  be  broader,  and  there  is  an  incipient  plication  on 
each  side  of  the  median  fold.  The  outline  is  still  broader  in 
S.  crispus,  becoming  decidedly  sub-elliptical,  and  the  two 
lateral  plications  on  the  dorsal  valve  are  nearly  equal  in 
strength  to  the  median  fold.  The  surface  ornamentation 
consists  of  fine  spinulose,  or  granulose,  concentric  strise, 
differing  very  little  in  any  of  the  three  species. 

In  tracing  the  development  of  R.  bicostata,  var.  petila, 
the  shell  is  found  to  retain  its  embryonic  characters  up  to 
full  growth,  neither  materially  changing  its  form,  nor  adding 
to  the  primitive  number  of  plications.  Likewise,  S.  crispus, 
var.  simplex,  changes  very  little  except  to  increase  in  width 
and  add  a  pair  of  plications  at  maturity.  Individuals  of 
S.  crispus  develop  parallel  to  the  variety  simplex,  up  to  a 
length  of  5  mm.,  or  until  about  two-thirds  the  size  of  full- 
grown  examples  is  attained.  Subsequently,  more  plications 
are  added,  increasing  the  number  from  three  or  five  to  eleven, 
but  otherwise  the  general  features  of  the  shell  are  unchanged. 
Even  the  relative  convexity  of  the  valves  remains  the  same 
at  all  periods. 

In  the  incipient  forms  the  cardinal  line  extends  for  about 
one-fourth  the  width  of  the  shell,  and  at  maturity  measures 
three-fourths  of  this  width.  The  foramen  does  not  develop 
at  the  same  rate ;  at  first  it  occupies  one-half  or  one-third  of 
the  ventral  area,  but  advancing  growth  gradually  diminishes 
this  ratio,  until  it  is  one-fourth  or  one-fifth  the  size  of  the 
hinge-area.  Two  narrow,  triangular,  deltidial  plates  are 
present  in  full-grown  individuals,  but  they  do  not  serve  to 
close  the  fissure,  which  remains  open  in  all  stages  of  growth. 

S.  crispus,  var.  simplex,  reaches  a  width  of  8  mm.,  and 
S.  crispus  often  measures  22  mm.  in  width.  Occasionally  a 
specimen  of  S.  crispus  of  the  usual  size  is  found  with  but 
seven  plications  on  the  dorsal  valve,  suggesting  a  very  large 
example  of  the  variety,  or  that  the  characters  of  the  smaller 
and  simple  form  are  sometimes  continued  far  beyond  the 
period  when  they  usually  disappear.  Also,  the  features  both 
of  the  species  and  variety  may  be  combined  in  a  single  speci- 


382  STUDIES  IN  EVOLUTION 

men,  as  one  abnormal  example  has  three  plications  on  one 
side  of  the  median  fold  and  four  on  the  other. 

Spirifer  radiatus  Sowerby,  1825. 

(PLATE  XX,  figures  9-11.) 

Spirifera  radiata  Hall.     Twenty-eighth  Ann.  Kept.  N.  Y.  State  Mus.  Nat. 

Hist.,  p.  157,  pi.  24,  figs.  20-30,  1879. 
Hall.     Eleventh  Ann.  Kept.  State  Geol.  Indiana,  p.  296,  pi.  24, 

figs.  20-30,  1882. 

The  series  of  specimens  representing  the  gradation  in  size 
from  very  young  to  mature  forms  is  quite  complete,  but, 
unfortunately,  the  characters  of  the  most  interesting  feature, 
the  deltidium,  are  not  well  shown.  The  foramen  is  usually 
but  partially  closed  when  the  shell  reaches  nearly  its  full 
dimensions,  and  at  this  period  the  beak  of  the  ventral  valve 
is  so  incurved  and  thickened  that  the  detailed  development 
of  the  deltidial  plates  is  obscured,  and  rendered  difficult  of 
interpretation. 

This  species  has  been  so  fully  discussed  in  all  its  aspects, 
on  account  of  its  wide  geographical  distribution  and  varied 
physical  conditions,  that  a  diagnosis  of  the  adult  form  is  un- 
necessary in  this  place  (vide  Plate  XX,  figures  11,  11  a). 

Incipient  Form  (Plate  XX,  figures  10,  10  a).  —  The  small- 
est example  yet  detected  has  a  length  of  1.5  mm.  The 
specimen  is  not  well  preserved,  and  the  one  used  for  illustra- 
tion and  description  is  somewhat  larger,  measuring  2  mm.  in 
length.  The  differences  appear  to  be  so  slight  that  the 
characters  of  the  larger  may  well  be  applied  to  the  smaller 
individual. 

The  shell  is  nearly  circular  and  flattened,  with  the  beaks 
not  incurved  but  directed  outward.  The  area  of  the  ventral 
valve  is  broad,  triangular,  open,  and  extends  nearly  the 
entire  length  of  the  cardinal  line.  The  incipient  dorsal  fold 
and  ventral  sinus  extend  nearly  to  the  beaks,  and  on  each 
side  there  are  about  ten  radiating  strise.  Radii  are  also 
present  on  the  fold  and  in  the  sinus. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     383 

Developmental  Changes. 

The  changes  in  the  shell  from  advancing  growth  are  prin- 
cipally the  gradual  widening  of  the  valves,  on  account  of  the 
extension  of  the  cardinal  line  and  extremities,  and  the  in- 
curving of  the  beaks,  from  the  progressive  increase  in  the 
depth  of  the  valves.  From  being  circular  in  outline,  the 
shell  slowly  widens  until  it  is  one-seventh  wider  than  long. 
The  ventral  beak  in  old  specimens  is  so  arched  over  the  area 
as  nearly  to  conceal  it,  and  prevent  the  opening  of  the  valves 
to  any  extent.  In  the  early  stages  the  depth  of  the  con- 
joined valves  is  about  half  the  length  of  the  shell,  while  in 
obese  mature  forms  the  depth  is  equal  to  the  length. 

The  deltidial  plates  first  appear  as  narrow  elevated  laminae 
along  the  sides  of  the  fissure  under  the  ventral  beak.  A 
specimen  about  half-grown  shows  them  as  represented  in 
Plate  XX,  figure  9,  consisting  of  triangular  plates  approxi- 
mately as  in  figure  3'  of  the  following  diagram.  They  are 
subsequently  united  along  their  inner  margins,  and  rarely, 
in  the  material  at  hand,  can  any  appearance  of  a  foramen  be 
discovered.  In  old  shells  the  growth  and  thickening  of 
the  pseudo-deltidium  makes  it  rugose,  and  it  nearly  closes 
the  area. 

From  an  examination  of  a  number  of  species  of  Spirifer 
showing  considerable  variety  in  the  mode  of  development  of 
the  pseudo-deltidium,  it  is  believed  that  there  is  no  essential 
difference,  and  that  all  intermediate  conditions  between  the 
features  represented  in  Spiriferina  by  Deslongchamps  (see 
Summary)  and  the  characteristic  mode  of  development  in 
Terebratula  and  RJiynchonella  occur  in  this  group.  The  genus 
Spirifer  presents  all  these  stages.  In  some  species  the  area 
is  apparently  closed  by  growth  from  the  apex,  and  in  others 
by  the  meeting  of  the  deltidial  plates  at  the  base  of  the  area 
and  inclosing  a  foramen  as  in  Rhynchonella.  Spirifer  niag- 
arensis,  S.  perlamellosus,  and  8.  cumberlandice  are  examples 
of  the  former  mode,  and  S.  sulcatus  and  approximately 
S.  radiatus  represent  the  latter.  Both  conditions  are  reached 


384  STUDIES  IN  EVOLUTION 

by  accretion  along  the  inner  edges  of  the  deltidial  plates. 
The  initial  state  is  represented  by  a  narrow  elongate  lamina 
on  each  side  of  the  triangular  area.  Further  growth  pro- 
duces a  triangular  plate,  and  to  the  form  of  the  triangle  is 
due  the  apparent  growth  of  the  pseudo-deltidium  from  the 
apex  of  the  fissure  downward,  or  from  the  base  of  the  fissure 
toward  the  beak  of  the  ventral  valve. 

The  accompanying  diagrammatic  outlines  (figure  129)  serve 
to  illustrate  the  changes  and  the  final  results. 

129 


A  A 


FIGURE  129.  —  Deltidial  development  in  Spirifer. 

Figure  1  represents  an  area  in  an  early  stage  of  growth, 
with  a  narrow  deltidial  plate  on  each  side,  alike  for  each 
series. 

Figure  2  shows  scalene  triangular  plates,  with  the  shortest 
side  at  the  base  of  the  area. 

Figure  2'  shows  plates  with  the  two  free  edges  more  nearly 
equal. 

Figure  2"  presents  narrow  triangular  plates  as  in  figure  2, 
but  with  the  shortest  edges  in  the  apex  of  the  area. 

In  figures  3,  3',  3",  the  growth  has  continued  in  the  direc- 
tion initiated  in  the  preceding  stage,  and  the  apex  of  the 
area  has  been  partially  filled  from  the  internal  thickening  of 
the  beak. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     385 

Figures  4,  4',  4"  show  the  completed  deltidial  plates  with 
the  circular  perforation.  The  plates  in  figure  4  nearly  close 
the  area,  while  in  figure  4"  the  opening  is  nearly  as  high  as 
wide.  Further  growth  can  now  take  place  only  along  the 
lower  free  edges  of  the  plates. 

Figure  5  represents  the  results  of  subsequent  growth  and 
thickening,  which  have  obliterated  the  evidences  as  to  the 
mode  of  development,  and  unified  all  three  cases.  The  posi- 
tion of  the  foramen  below  the  apex  of  the  area  does  not  appear 
to  be  due  to  the  approximation  and  union  of  the  deltidial 
plates,  but  to  the  lowering  of  the  actual  cavity  of  the  beak 
from  the  natural  thickening  of  the  shell,  so  that  the  foramen, 
as  in  other  genera,  is  at  the  real  termination  of  the  ventral 
umbonal  cavity. 

It  is  seen  that  the  manner  of  development  is  alike  in  each 
case,  varying  only  from  differences  in  the  form  of  the  plates 
in  the  earlier  stages.  The  finished  pseudo-deltidium  is  also 
the  same,  although  the  methods  of  attaining  the  result  differ 
in  each. 

Figures  1,  2,  3,  4  are  represented  by  S.  sulcatus,*  and  vary 
in  no  important  particulars  from  the  mode  of  development  in 
Terebratula  and  Rhynchonella. 

Figures  1,  2',  3',  4'  are  partially  represented  by  S.  radiatus, 
although  in  this  species  the  circular  foramen  is  usually  oblit- 
erated by  subsequent  thickening  and  growth.  (See  figures 
9,  10,  11,  Plate  XX.) 

Figures  1,  2",  "3"  are  well  shown  in  S.  niagarensis  (figure 
8,  Plate  XX),  and  the  subsequent  stages  appear  in  mature 
forms  of  S.  perlamellosus  and  S.  cumberlandice.  Other  forms, 
notably  those  with  elevated  areas  (such  as  S.  macronotus, 
S.  medialis,  together  with  Cyrtina  and  Cyrticf),  present  con- 
siderable differences  in  the  completed  pseudo-deltidium,  due 
principally,  it  is  believed,  to  the  internal  thickening  of  the 
beak  and  the  growth  of  the  transverse  septum. 

The  pseudo-del tidium  of  Spirifer  thus  appears  to  be  the 

*  State  of  New  York,  Report  of  the  State  Geologist  for  the  year  1882,  pub- 
Hshed  1883,  pi.  ix,  figs.  1,  2,  3. 

25 


386  STUDIES  IN  EVOLUTION 

exact  homologue  of  the  deltidial  plates  in  Terebratula,  RJiyn- 
chonella,  etc.,  and  to  be  radically  different  from  the  deltidium 
of  Strophomena,  Stropheodonta,  Orthothetes,  and  allied  genera. 


SUMMARY  OF  DEVELOPMENTAL  CHANGES. 

Size  and  Contour.  —  Although  the  species  described  in  the 
preceding  pages  present  a  wide  variation  in  form  and  general 
appearance,  the  nature  of  the  changes  which  take  place  in  the 
development  of  the  shell  is  remarkable  in  its  uniformity. 

In  nearly  every  species  the  inceptive  state  is  represented 
by  a  shell  having  a  sub-circular  outline,  with  valves  of  slight 
convexity.  This  phase  usually  disappears  before  the  indi- 
vidual reaches  a  length  of  1  mm.,  after  which  the  specific 
characters  are  assumed,  and  are  progressively  emphasized 
with  each  succeeding  increment. 

On  comparing  the  incipient  stage  in  these  fossil  shells 
with  that  of  recent  brachiopods,  as  given  by  Mr.  E.  S.  Morse 
for  Terebratulina  and  by  Mr.  W.  K.  Brooks  for  G-lottidia, 
it  is  found  that,  in  respect  to  actual  size,  there  is  a  slight, 
though  perhaps  unessential  difference.  At  the  earliest  stage 
of  growth  figured  by  Morse,*  the  shell  has  a  length  of  about 
.3  mm.  and  in  the  next  stage  represented,  of  approximately 
.6  mm. 

The  first  two  stages  of  the  shell  figured  by  Mr.  Brooks  f 
represent  free  animals,  and  measure  .24  and  .3  mm.  in 
length,  respectively.  The  shell  became  attached  by  the 
pedicle  only  upon  attaining  a  length  of  2.5  mm. 

Most  of  the  fossil  forms  have  furnished  evidence,  either 
from  actual  elemental  specimens  or  from  the  apical  portions 
of  subsequent  incipient  stages,  that  the  true  initial  shell  did 
not  reach  a  size  of  more  than  .5  mm.  in  length.  Soon  after 

*  On  the  Early  Stages  of  Terebratulina  septentrionalis.  Mem.  Boston  Soc.  Nat. 
Hist.,  II,  pi.  i.  figs.  2,  3,  1869. 

t  The  Development  of  Lingula  and  the  Systematic  Position  of  the  Brachiopoda. 
Johns  Hopkins  University,  Chesapeake  Zob'l.  Lab.,  pis.  i  and  ii,  1879. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     387 

this  period  the  characters  of  each  species  become  developed 
and  impressed  upon  the  shell  more  or  less  gradually. 

Even  such  distinct  groups  as  Camarot&chia,  Spirifer,  Athy- 
ris,  Rhynchotreta,  Anastrophia,  Nudeospira,  and  the  meris- 
toids,  in  their  initial  stages,  approach  one  another  so  closely 
that  they  can  be  determined  only  from  comparatively  trivial 
features.  They  are  alike  in  form,  contour,  convexity,  beaks, 
and  cardinal  area,  and  the  only  marked  differences  are  to  be 
found  in  the  faint  indications  of  plications,  strias,  folds,  and 
sinuses.  For  species  of  some  genera,  as  Dalmanella,  Rhipid- 
omella,  Meristina,  and  Spirifer,  even  these  characters  are 
not  determinative,  and  it  is  impossible  to  refer  certain 
embryos  to  their  proper  places. 

From  the  foregoing  statements  it  would  be  naturally 
inferred  that  the  species  which  at  maturity  present  char- 
acters abnormal  to  the  typical  structure,  have  been  diverted 
from  the  harmony  which  existed  in  the  incipient  stages,  with 
the  other  members  of  the  group.  This  has  been  shown  to  be 
the  case  in  all  the  foregoing  reversed  species  examined,  belong- 
ing to  the  genera  Anastrophia,  Strophonella,  and  Mimulus. 

Beginning  with  the  initial  shell  having  a  circular  outline 
and  depressed  valves,  it  is  found  that  subsequent  growth  takes 
place  about  the  periphery,  and  in  the  majority  of  species  the 
convexity  is  gradually  increased  until  maturity  is  reached. 
This  assertion  does  not  hold  true  for  such  forms  as  the 
StrophomenidaB,  which  vary  in  convexity,  either  very  slowly 
or  not  at  all,  up  to  individuals  about  half -grown,  when  the 
valves  become  more  or  less  deflected  and  often  concave. 
Such  reversion  in  the  shell  is  in  conformity  with  the  de- 
generacy which  is  traced  in  the  development  of  the  cardinal 
area  and  pedicle-sheath,  mentioned  on  a  subsequent  page. 

The  observations  of  Brooks  and  Morse,  in  the  works  cited, 
show  that  in  both  the  hingeless  and  the  hinged  brachiopods, 
as  represented  by  Linyula  and  Terebratulina,  the  early 
stages  of  the  shell  approach  a  sub-circular  outline,  and  Brooks 
remarks  (op.  cit.  p.  43),  that  "  the  recent  and  fossil  shells  of 
the  various  species  of  Crania,  Lingula,  Lingulella,  and  Obolus, 


388  STUDIES  IN  EVOLUTION 

and  other  hingeless  brachiopods,  furnish  a  series  of  adult 
forms  representing  all  the  changes  through  which  the  outline 
of  Lingula pyramidata  passes  during  its  development." 

In  these  respects,  then,  uniformity  is  established  in  the 
embryology  of  the  ancient  Silurian  types  and  their  modern 
descendants. 

Valves.  —  The  dorsal  valve  in  young  shells  is  smaller  than 
the  opposite,  and  usually  more  depressed.  These  relations, 
as  a  rule,  are  continued  up  to  adult  size,  except  that  the 
ventral  valve  often  increases  more  rapidly  in  convexity,  pro- 
ducing a  consequent  incurving  of  the  beak  over  the  cardinal 
area ;  as  in  Dalmanella  and  CamarotoecTiia  indianensis.  Some 
species  present  both  beaks  as  incurved,  a  condition  well 
represented  in  Meristina  Maria,  Dictyonella  reticulata,  Cam- 
arotoechia  acinus,  and  C.  neglecta.  In  Anastrophia  the  com- 
parative relations  of  the  valves  become  reversed  from  their 
initial  condition,  on  account  of  the  more  rapid  increase  in 
the  depth  of  the  dorsal  valve,  so  that,  at  maturity,  the  dorsal 
beak  is  much  incurved,  and  often  the  umbo  extends  beyond 
that  of  the  other  valve,  although  the  beaks  preserve  their 
normal  condition  of  superposition. 

Several  of  the  species  show  an  embryonal  sinus  in  the 
dorsal  valve,  with  a  corresponding  fold  in  the  ventral,  begin- 
ning soon  after  the  initial  stage  of  the  shell  has  been  passed, 
and  disappearing  before  the  shell  is  half  grown.  Those  forms 
presenting  this  feature  to  a  marked  degree  are  Rhynchotreta 
cuneata,  Oamarotoechia  Whitii,  C.  indianensis,  C.  neglecta, 
O.  acinus,  Atrypa  reticularis,  and  Homceospira  sobrina.  In 
Rhynchotreta  cuneata  and  Atrypa  reticularis  (Plate  XVIII, 
figures  12-15,  and  Plate  XX,  figures  12-14),  the  gradual  in- 
ception of  this  sinus,  its  maximum  development  and  obsoles- 
cence, and,  finally,  its  reversion  into  a  fold  which  thereafter 
persists  and  usually  increases  in  prominence  in  all  the  suc- 
ceeding stages  of  growth,  have  been  shown.  The  embryonal 
sinus  is  not  present  in  Spirifer,  Anastrophia,  Dictyonella, 
Meristina,  Whitfieldella,  Dalmanella,  Rhipidomella,  nor  in  the 
StrophomenidaB.  Such  of  these  as  show  a  dorsal  fold  or  ven- 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     389 

tral  sinus  have  them  developed  early  in  the  growth  of  the 
shell,  and  they  usually  increase  regularly  to  the  time  when 
the  full  size  of  the  shell  is  attained. 

Beaks.  —  The  beak  of  the  ventral  valve  in  its  earliest  con- 
dition is  commonly  erect,  pointed  outward,  and  of  a  broad 
triangular  form,  while  that  of  the  dorsal  valve  is  small,  not 
prominent,  and  lies  in  the  longitudinal  axis  of  the  shell. 
In  all  cases  the  subsequent  deepening  of  the  valves  tends  to 
incurve  the  beaks  toward  the  cardinal  area.  The  degree  of 
incurvature  varies  greatly  in  the  different  species.  Meristina 
rectirostris  shows  a  minimum,  and  Meristina  Maria  or  Atrypa 
reticularis  a  maximum,  and  between  these  limits  all  inter- 
mediate conditions  occur.  The  usual  degree  of  incurvature 
is  presented  in  Spirifer  radiatus,  Homceospira  evax,  and  the 
Rhynchonellidse. 

The  outlines  on  Plate  XVIII,  illustrating  the  profiles  of 
the  beaks  in  a  series  of  RhyncJiotreta  cuneata,  represent  an 
uncommon  condition,  for  in  this  species  the  ventral  beak, 
from  its  divergent  initial  position,  gradually  approaches,  and 
at  maturity  attains  parallelism  with  the  longitudinal  axis  of 
the  shell.  It  never  becomes  sufficiently  incurved  to  conceal 
to  the  slightest  degree  the  deltidial  area,  while  the  initial 
dorsal  beak  becomes  more  and  more  incurved,  until,  finally, 
it  lies  entirely  within  the  ventral  umbonal  cavity. 

Those  species  furnished  with  a  circular  apical  perforation, 
as  Atrypa  reticularis,  Homceospira  evax,  and  RhyncJiotreta 
cuneata,  lose  the  initial  point  of  the  ventral  beak  from  absorp- 
tion, due  to  the  increase  in  the  size  of  the  perforation  or  to 
its  final  terminal  position.  In  Atrypa  reticularis,  or  Meris- 
tina Maria  even,  both  beak  and  perforation  are  destroyed, 
from  the  forcing  of  the  ventral  beak  into  contact  with  the 
dorsal  umbo,  produced  by  the  great  increase  in  the  depth  of 
the  valves  from  growth  along  their  anterior  margins. 

Cardinal  Area.  —  Omitting  for  the  present  the  Stropho- 
menidse  and  Orthidse,  the  initial  state  of  the  ventral  cardinal 
area  for  all  other  forms  is  a  broad  triangular  opening  beneath 
the  beak,  with  simple  sharp  margins.  This  condition  is 


390  STUDIES  IN  EVOLUTION 

never  passed  by  Meristina  rectirostris,  which  shows  a  uni- 
form, open,  triangular  area  in  every  period  of  growth. 

A  further  advanced  state  of  progress  initiates  the  deltidial 
plates,  which  first  appear  as  narrow  laminae  along  the  sides 
of  the  area.  The  areal  development  of  Spirifer  crispus, 
Camarotoechia  neglecta,  and  C.  acinus  ceases  at  this  point. 

In  the  next  stadium  the  further  growth  of  the  deltidial 
plates  along  their  free  edges  gives  them  a  triangular  form, 
and  they  tend  to  narrow  the  limits  of  the  opening  and  define 
the  peduncular  foramen.  Spirifer  niagarensis  and  Camaro- 
toechia Whitii  represent  species  which  are  arrested  at  this  period. 

The  completed  growth  shows  the  deltidial  plates  uniting 
by  symphysis  along  a  median  line,  and  enclosing  near  the 
apex  of  the  area  a  more  or  less  circular  pedicle  perforation. 
Rhynchotreta  cuneata,  Meristina  Maria,  Homoeospira  evax, 
etc.,  after  passing  through  all  the  earlier  conditions,  reach 
this  limit  of  development. 

The  results  of  senile  and  extravagant  growth  often  oblit- 
erate or  degenerate  the  normal  deltidial  advancement,  the 
plates  becoming  thickened  and  their  features  obscured,  while 
in  some  species  processes  are  given  off,  as  in  a  number  of  the 
Mesozoic  Rhynchonellidse. 

The  cardinal  area  of  the  Strophomenidse  in  its  early  phase 
shows  a  small  pedicle-sheath  for  the  ventral  valve  and  a 
narrow  grooved  process  under  the  beak  of  the  dorsal.  The 
perforation  for  the  passage  of  the  peduncle  does  not  mate- 
rially increase  in  size  with  the  growth  of  the  shell  and  often 
is  obliterated,  while  the  dorsal  callosity  usually  reaches  a 
considerable  development. 

Additional  evidence  of  the  degeneracy  of  the  pedicle  is 
afforded  by  many  species  of  other  genera,  which  have  a 
calcareous  attachment  to  foreign  objects  at  the  apex  of  the 
ventral  valve,  the  pedicle,  therefore,  becoming  functionally 
obsolete. 

Observations  having  some  analogy  with  the  facts  here  pre- 
sented have  been  made,  in  a  very  restricted  sense  and  usually 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOFODA     391 

incidentally,  by  various  authors.  The  present  results,  though 
derived  from  the  species  of  a  single  fauna,  must  not  be  given 
too  limited  an  application,  for  they  involve  nearly  every 
important  family  of  Paleozoic  articulate  brachiopods,  and 
it  may  be  tentatively  assumed  that,  as  a  rule,  the  essential 
features  of  variation  observed  in  any  member  of  a  genus 
will  hold  good  of  the  other  members.  In  regard  to  the 
development  of  the  characters  of  the  pedicle-passage,  i.e., 
the  deltidial  plates  and  the  foramen,  there  is  good  reason 
to  regard  the  process  as  substantially  identical  in  all  the 
genera  represented,  making  the  necessary  allowance  for 
the  peculiar  variation  seen  in  the  Strophomenidse,  which 
may  not,  however,  prove  it  an  exception  to  the  general 
statement.  * 

The  various  terms  which  have  been  sometimes  applied  to 
the  condition  of  the  deltidial  plates  in  the  rostrate  genera  —  as 
deltidium  amplectens,  when  the  foramen  is  entirely  surrounded 
by  the  plates,  as  in  various  Mesozoic  Rhynchonellse  (but  in 
no  Paleozoic  species,  as  far  as  known);  deltidium  sedans, 
when  the  plates  bound  the  foramen  only  on  the  lower 
side,  the  upper  side  encroaching  on  the  substance  of  the 
umbo,  as  in  Terebratula  Whitfieldella,  etc. ;  deltidium  dis- 
cretum,  when  the  plates  do  not  come  into  contact,  as  in 
Terebratetla,  some  species  of  Rhynchonella,  etc.,  —  must  be 
regarded  as  having  no  further  significance  than  to  express 
the  existing  condition  of  the  foramen  and  deltidial  plates 
in  any  given  specimen;  that  is,  as  indicating  a  stage  of 
development,  not  necessarily  a  generic  or  even  specific 
character. 

The  observations  of  M.  Eugene  Deslongchamps  upon  these 
features  are  of  much  value,  and  in  most  respects,  as  far  as 
carried  out  upon  related  forms,  are  in  harmony  with  those 
here  expressed  (Note  sur  le  developpement  du  deltidium  chez 
les  brachiopodes  articule's :  Bull.  Soc.  Geol.  France,  2e  ser.  t. 
xix,  pp.  409-413,  pi.  ix,  1862),  but  with  his  conclusions 

*  See  footnote,  p.  393. 


392  STUDIES  IN  EVOLUTION 

there  are  some  points  of  difference.  The  investigations 
referred  to  were  made  upon  one  (or  more)  Mesozoic  species  of 
Terebratula,  Rhynchonella,  and  Spiriferina,  specific  designa- 
tions not  given.  The  illustrations  of  Terebratula  (figures 
1  a,  b,  c,  and  column  A,  a,  /3,  7,  S)  show  in  effect  the  char- 
acters seen  in  Whitfieldella  nitida,  Meristina  Maria,  and 
others ;  those  of  Rhynchomlla,  early  stages  of  similar  charac- 
ter, resulting  in  a  deltidium  amplectens,  such,  as  just  observed, 
have  not  been  found  in  Paleozoic  Rhynchonellse. 

In  Spiriferina,  according  to  M.  Deslongchamps,  the 
pseudo-deltidium  is  produced  by  the  gradual  development 
of  a  single  plate  in  the  apex  of  the  triangular  opening,  in- 
creasing downward  with  age,  a  very  distinct  mode  of  for- 
mation from  all  the  others,  and  open  to  verification  in  the 
species  described  by  that  author,  as  his  figures  make  no 
allowance  for  a  pedicle-sinus  or  perforation,  a  feature,  though 
not  of  frequent  occurrence  in  the  Spiriferidse,  yet  one  neces- 
sary to  account  for. 

The  writers  have  examined  specimens  of  Spiriferina  pinguis 
Deslongchamps,  S.  rostrata  Schlotheim,  and  S.  Walcotti  Sow- 
erby,  and  find  that  these  species,  at  least,  develop  triangular 
deltidial  plates.  Those  in  Spiriferina  pinguis  and  S.  Walcotti 
are  comparable  with  the  same  parts  in  Spirifer  perlamellosus 
and  £  cumberlandice,  and  their  form  and  mode  of  growth  are 
expressed  by  the  outlines  2",  3",  on  page  384,  and  is  further 
shown  in  figure  130.  Additional  growth  causes  the  plates  to 
unite  along  the  median  line,  obliterating  the  partially  formed 
pedicle-perforation,  and  subsequent  increment  can  naturally 
take  place  only  along  their  lower  free  edges. 

The  remarks  on  Spirifer  radiatus  and  S.  crispus  indicate 
that  the  development  of  the  plates  in  this  member  of  the  same 
family  is  quite  in  harmony  with  the  process  as  seen  in  the 
rostrate  forms  generally. 

The  following  is  the  summarization  of  Deslongchamps's 
conclusions,  as  given  by  himself: 

(1)  The  deltidium  is  one  of  the  most  important  features 
in  the  articulated  brachiopods. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     393 

( 2 )  As  far  as  Jurassic  species  are  concerned,  the  deltidium 
may  suffice  to  characterize  the  families. 

(3)  In  the  various  stages  of  development  of  this  part  the 
aspect  of  the  shell  is  entirely  changed. 

(4)  The  deltidium  appears  under  three  important  modifi- 
cations :  (A)  Development  below  the  peduncular  arm,  char- 
acterizing the  Terebratulidse ;    (B)   development  above   the 
peduncle,  Spiriferidae ;  (C)  mixed  development,  surrounding 
the  peduncle,  Rhynchonellidae. 

(5)  The  stage  at  which  the  development  is  arrested  or  the 
exuberance  of  development 

may  suffice  to  characterize  13° 

sections  under  the  families. 

It  has  just  been  shown 
that  conclusions  2,  4,  and 
5  are  not  capable  of  the 
extended  application  which 
Deslongchamps  has  given 
them. 

A  preceding  remark,     FlGDRE  130.  — Deitidial  development  in 

that  the  COUrse  of  develop-          -?,  ^1  Spiriferina  pinguis  Deslougchamps ; 
n  ,1      j    -,.•  -i  •    i    -I  3,  4>  Spiriferina  Walcotti.  Sowerby; 

mentofthedelMialcharao-       £  ^4a  rostmta  Schlotheimy 
ters  throughout  the  genera 

here  discussed  may  be  considered  as  fundamentally  uniform, 
calls  for  explanation  in  its  application  to  the  Orthidse  and  the 
Strophomenidse.*  In  the  latter  forms  it  has  been  shown  that 
the  remarkable  development  of  the  pedicle-sheath  is  primary, 
and  is  invariably  more  or  less  atrophied  with  age,  and  probably 
functionally  inactive  at  maturity.  Hence  the  retention  of 
this  sheath  in  any  species  at  maturity  is  the  perdurance  of 
what  must  serve  as  an  embryonic  character  within  the  limits 
of  this  family.  It  cannot  escape  observation  that  the  pedicle- 
sheath  is  in  analogy  with  the  entire  rostrate  umbo  of  the 

[*  The  following  correlations  (supra)  of  the  characters  of  the  cardinal  area 
were  made  before  the  true  significance  of  the  pedicle-sheath  in  the  strophomenoid 
genera  was  understood.  The  subject  is  fully  discussed  (ante)  in  the  second  part 
of  the  Development  of  the  Brachiopoda.] 


394  STUDIES  IN  EVOLUTION 

ventral  valve  in  the  Rhynchonellae,  Terebratulse,  etc.,  as  a 
specialized  extension  of  the  valve  for  the  protrusion  of  the 
pedicle.  (Compare  the  extreme  development  of  the  umbo 
in  the  genus  Terebrirostra.)  That  these  parts  are  also  homo- 
logues,  it  is  difficult  to  prove,  on  account  of  the  pedicle- 
sheath  becoming  more  degenerate  as  maturity  approaches ; 
but,  assuming  this  homology,  the  sheath  and  its  gradual  dis- 
appearance may  be  regarded  as  an  indication  of  degeneracy  in 
the  family,  the  presence  of  the  sheath  pointing  toward  a 
derivation  from  the  rostrate  type. 

The  atrophy  of  an  organ  so  highly  specialized  as  the  sheath 
is,  aside  from  any  consideration  of  relationship  to  other 
groups  of  the  brachiopods,  itself  confirmatory  of  such  de- 
generacy. Furthermore,  it  will  be  noticed  that  there  is, 
throughout  these  strophomenoids,  an  inclination,  as  mature 
growth  comes  on,  toward  the  simple  triangular  pedicle- 
apertures  in  Orthis.  The  disappearance  of  the  pedicle-sheath 
leaves  the  aperture  of  the  ventral  valve  essentially  free,  as 
seen  in  Leptcena  rhomboidalis  and  Orthothetes  subplanus, 
while  the  aperture  of  the  dorsal  valve  is  filling  pari  passu 
with  a  callosity.  In  other  words,  the  structure  of  these 
parts  is  actually  degenerating  toward  maturity,  to  that  of 
Orthis,  which  is  the  simplest,  least  differentiated  condition 
among  the  articulated  brachiopods,  and  serves  to  fortify  the 
position  of  the  genus  at  the  base  of  the  entire  series.  In 
Orthis,  the  pedicle-apertures  on  both  valves  are  of  the  same 
size  in  early  growth,  and  have  undoubtedly  acted  together 
as  a  single  opening,  through  which  the  fleshy  arm  was  pro- 
truded as  much  on  one  side  as  the  other,  a  fact  indicative  of 
an  extreme  lack  of  differentiation  in  the  two  valves  in  the 
articulate  species,  but  agreeing  closely  with  some  of  the 
inarticulate  genera,  as  Lingula,  Leptobolus,  Obolus.  The  spe- 
cialization which  accompanies  subsequent  growth  confines 
the  pedicle  more  closely  to  the  ventral  aperture,  and,  as  a 
result,  the  dorsal  aperture  is  gradually  filled  by  a  callosity. 
Thus,  also,  the  Strophomenidae ;  but  Orthothetes  subplanus 
shows  at  maturity  what  has  not  yet  appeared  in  Dalmanella; 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     395 

namely,  the  initiation  of  deltidial  plates,  in  conformity  with 
the  general  course  of  development  of  the  cardinal  features 
observed  in  other  families. 

It  is  not  well  in  this  place  to  go  beyond  the  scope  of  this 
work,  and  the  species  of  Strophomenidse  here  discussed  for 
facts  confirmatory  of  these  observations.  It  maybe  remarked 
that  the  stage  at  which  the  development  of  the  deltidial 
features  has  been  arrested  at  maturity  in  this  family  varies 
with  the  species,  not  with  the  genus.  When  every  trace  of 
these  features  is  obliterated,  as  is  usual  in  Stropheodonta,  a 
slight  abrasion  of  the  apical  substance  of  the  shell  will  often 
show  a  trace  of  the  obsolete  pedicle-tube.  At  times,  in  the 
same  genus,  this  is  retained  at  maturity  as  an  external  feature, 
and  in  such  a  case  is  usually  accompanied  by  some  indication 
of  the  sub-apical  sheath.  In  both  StropJieodonta  and  Ortho- 
thetes  (especially  of  the  later  Paleozoic  faunas),  the  cavity 
of  the  pedicle-sheath,  if  it  is  retained  in  any  form,  at  matu- 
rity, has  been  filled  by  the  deposition  of  calcareous  matter 
about  the  compound  cardinal  processes  of  the  opposite  valve, 
and  thus  wholly  diverted  from  its  original  function. 

In  conclusion,  it  is  to  be  observed  that  of  recent  species  of 
brachiopods  a  very  great  number  show  an  incomplete  devel- 
opment of  the  deltidial  plates  at  maturity.  Such  is  Rhyncho- 
nella  to  a  large  degree  ;  also  Cistella,  Kraussina,  Terebratella, 
and  Magasella ;  and  it  may  be  assumed  that  the  structural 
degeneracy  which  is  thus  indicated  is  the  natural  concomitant 
of  the  secular  decline  of  the  entire  class. 

It  is  not  improbable  that  from  an  early  form  related  to  the 
genus  Orthis,  phylogenetic  development  tended  in  two  main 
channels,  —  one  leading  through  Strophomena,  Scenidium, 
Orthisina,  Leptcena,  Ohonetes,  Productus,  and  Strophalosia, 
and  the  other  in  the  direction  of  Rhynchonella,  Spirifer, 
Atrypa,  Retzia,  and  Terebratula. 

Internal  Apparatus.  — The  present  observations  upon  the  de- 
velopment of  the  brachial  supports  are  limited  to  the  species 
Homoeospira  evax.  Here  it  is  found  that  the  number  of  revo- 
lutions of  the  spiral  ribbon  increases  with  age,  but  it  is  not 


396  STUDIES  IN  EVOLUTION 

certain  what  the  inceptive  condition  of  this  apparatus  may 
have  been.  In  the  early  stage  represented  on  page  365, 
where  the  ribbon  has  completed  two  revolutions,  the  sup- 
ports must  have  been  exceedingly  tenuous  and  delicate,  for 
they  can  be  traced  in  the  crystalline  or  muddy  filling  of  the 
shell  only  by  extremely  faint  lines,  composed  of  minute  dots 
of  pyrite.  As  observed  under  the  discussion  of  these  features, 
the  character  or  actual  existence  of  the  loop  connecting  the 
spirals  was  not  established,  but  it  is  developed,  with  all 
normal  characters,  in  a  shell  4  mm.  in  length,  where  the 
ribbon  makes  four  revolutions. 

It  has  been  shown  by  Morse,*  that  in  Terebratulina  septen- 
trionalis  the  loop  (i.  e.,  the  entire  brachial  support)  begins 
by  the  development  of  two  acute  processes  from  the  lower 
moiety  of  the  dental  plate,  which  assume  the  character  of 
crura,  eventually  meeting  and  coalescing  on  the  dorsal  side, 
forming  the  completed  loop  at  an  early  stage,  the  ventral 
horns  of  the  loop  never  uniting.  The  simple  nature  of  the 
support  in  these  shells  precludes  the  possibility  of  the  con- 
tinued growth  which  obtains  in  the  more  complicated  appara- 
tus of  the  spiriferous  species.  The  inception  of  the  brachial 
support  was  observed  by  Morse  in  an  individual  1  mm.  in 
length,  but  the  lateral  processes  are  not  conspicuously  devel- 
oped until  a  length  of  3  mm.  is  attained,  and  they  have  not 
united  at  a  length  of  4  mm.  It  is  therefore  possible,  from 
these  data,  that  Homwospira  evax  does  not  have  the  loop 
completed  at  so  early  an  age  as  that  indicated  by  a  length 
of  2.5  mm. 

The  observations  by  Morse  are  corroborated  by  those  of 
Dall  f  on  Leiotliyrina  cubensis. 

Surface  Ornaments.  —  Nearly  all  the  observations  upon 
initial  shells  or  upon  that  portion  situated  at  the  apex  of  the 
beak  of  more  advanced  stages  and  representing  the  initial 

*  On  the  Early  Stages  of  Terebratulina  septentrionalis.  Mem.  Boston  Soc.Nat. 
Hist.,  II,  pi.  ii,  figs.  48-55,  1869. 

t  Report  on  the  Brachiopoda  of  Alaska  and  the  adjacent  Shores  of  North- 
west America.  Proc.  Acad.  Nat.  Sci.  Phila.,  1877,  pt.  ii,  p.  155. 


DEVELOPMENT  OF  SOME  SILURIAN  BRACHIOPODA     397 

shell,  seem  to  warrant  the  assertion  that  the  surface  orna- 
ments do  not  appear  until  the  second  or  a  later  period  is 
reached  in  the  development  of  the  shell. 

For  the  plicate  species  nearly  the  full  number  of  plica- 
tions appear  simultaneously,  as  in  Camarotoechia  indianensis, 
C.  acinus,  and  Rhynchotreta  cuneata,  or  they  are  introduced 
in  pairs,  as  upon  Camarotoechia  Whitii,  ?C.  neglecta,  and 
Homceospira  sobrina. 

The  striae  of  Leptcena  rhomboidalis  are  developed  to  the 
full  capacity  of  the  marginal  area  as  soon  as  the  first  growth- 
line  is  completed.  The  number  is  increased  in  three  distinct 
manners:  (a)  by  division,  (b)  by  intercalation,  and  (c)  by 
addition  at  the  cardinal  angles.  Some  species  present  all 
three  of  these,  while  others  add  to  their  striae  or  plications  by 
any  one  or  two  of  the  modes. 

The  concentric  ornamentation  in  such  species  as  Spirifer 
crispus  and  Orthothetes  subplanus  appears  early  in  the  growth 
of  the  embryo,  and  continues  to  be  repeated  without  varia- 
tion, except  in  Leptcena  rhomboidalis  and  allied  forms,  which 
develop,  during  the  last  stage  of  growth,  a  geniculated  cur- 
tain without  concentric  undulations. 

Varieties  and  Abnormalities.  —  Varieties  usually  begin  to 
express  themselves  early  in  the  development  of  the  shell, 
and  the  divergence  from  the  normal  form  rapidly  increases  as 
maturity  approaches.  Several  of  the  species  represented  by 
abundant  material  are  readily  separable  into  three  distinct 
groups  of  forms,  (a)  long  form,  (b)  normal  form,  and  (c)  broad 
form.  The  history  of  each  may  be  clearly  traced,  and  they 
usually  are  found  to  unite  with  the  line  of  the  normal  form 
(b)  several  removes  from  the  initial  member  of  the  series. 
Sometimes  the  varieties  do  not  reach  the  adult  dimensions  of 
the  normal  shell  and  may  be  considered  as  varietal  dwarfs. 

The  representation  of  varietal  and  of  certain  abnormal  con- 
ditions by  complete  series  of  fossil  specimens  shows  that  in 
these  directions  there  was  a  distinct  developmental  tendency, 
or  genetic  impulse,  independent  of  normal  growth. 

Senility  is  always  expressed  by  the  thickening  of  the  shell 


398  STUDIES  IN  EVOLUTION 

which  takes  place  after  the  individual  reaches  adult  size. 
The  thickening  may  involve  the  whole  interior  of  the  valves, 
producing  a  truncate  appearance  at  the  margins,  or  it  may 
take  place  by  frequent  interrupted  growth  along  the  margins, 
giving  to  this  portion  a  varicose  aspect.  As  a  result  of  this 
senile  growth,  the  vertical  diameter  of  the  shell  is  increased, 
and  the  beaks  are  involuted,  so  that  they  are  often  so  closely 
appressed  as  to  conceal  the  cardinal  area  and  truncate  the 
ventral  beak,  and  in  addition,  the  margins  of  the  valves  lose 
the  characteristic  ornamentation  of  the  species  and  corre- 
spond to  the  gerontic  stages  as  defined  by  Mr.  Hyatt.* 

Abnormalities  frequently  find  an  explanation  in  some  path- 
ological or  accidental  conditions  which  become  instituted  at 
any  period  in  the  life  of  the  animal,  and  leave  their  impress 
on  the  shell.  The  functional  failure  of  a  developing  organ 
may  cause  the  parts  to  revert  to  an  embryonal  type,  and 
although  it  is  difficult  to  apply  this  statement  to  the  shelly 
covering  of  the  animal,  yet  this  condition  is  sometimes  found. 
The  specimen  of  Oamarotoechia  neglecta  described  on  page  342 
is  an  instance  of  this  kind.  Another  abnormal  variation  is 
noticed  in  certain  individuals  which  preserve  the  larval  fea- 
tures of  the  shell  long  after  it  has  passed  the  early  stages, 
and  when,  in  many  cases,  it  has  reached  the  full  adult 
dimensions. 

*  Values  in  Classification  of  the  Stages  of  Growth  and  Decline,  with  proposi- 
tions for  a  New  Nomenclature.  Proc.  Boston  Soc.  Nat.  Hist.,  XXIII,  1888. 


5.   DEVELOPMENT  OF  BILOBITES* 

(PLATE  XXIII) 

THE  Linnsean  species  so  well  known  under  the  name  of 
Orthis  biloba,  and  so  widely  distributed  in  the  Silurian  rocks 
of  the  world,  represents  one  of  the  very  distinct  members 
into  which  the  Orthis  group  is  now  divided.  It  is  much 
removed  from  ordinary  Orthis  in  general  external  features, 
and  only  by  means  of  developmental  characters  is  it  possible 
to  arrive  at  any  idea  of  its  genetic  history. 

After  having  been  referred  to  various  genera,  including 
Anomia,  Terebratula,  Delthyris,  and  Spirtfer,  by  different 
authors  prior  to  1848,  Davidson  f  first  showed  conclusively, 
from  a  study  of  the  internal  characters,  that  the  true  rela- 
tions were  with  the  genus  Orthis.  Its  position  has  since 
remained  unchallenged,  and  subsequent  investigation  has  not 
brought  forth  any  new  characters,  nor  invalidated  the  results 
obtained  by  Davidson.  The  additional  observations  here 
made  concerning  the  development  of  the  shell,  while  adding 
to  a  knowledge  of  the  species,  merely  serve  to  bind  this 
form  more  closely  to  the  group  having  the  broad  designa- 
tion of  Orthis.  Professor  King  in  1850  J  proposed  the  genus 
Dicoelosia  for  this  species,  on  account  of  its  characteristic 
form,  and  authors  disposed  to  divide  Orthis  have  recognized 
this  name.  Since  then  it  has  been  shown  that  Linne*  gave 
the  generic  term  .Bilobites  to  the  type  species  of  King's  genus, 
and  this  name  is  now  generally  adopted  with  the  rank  of  a 
sub-genus.  The  validity  of  the  specific  names  applied  to 

*  Amer.  Jour.  Sci.  (3),  XLII,  51-56,  pi.  i,  1891. 

t  Bull.  Soc.  Geol.  France,  2d  ser.,  V,  321,  t.  3,  fig.  18,  1848. 

J:  Monograph  Permian  Fossils,  106,  1850. 


%v 


400  STUDIES  IN  EVOLUTION 

variations  from  the  typical  form  is  not  of  much  moment  in 
this  place,  although  the  geologic  history  and  interpretation 
of  these  differences  are  of  considerable  interest.  Two  well- 
defined  varieties  or  species  are  recognized  in  Sweden,  and  are 
represented  in  outline  by  figures  2  and  28,  Plate  XXIII. 
The  prevailing  form  in  the  Wenlock  shales  at  Dudley,  Eng- 
land, agrees  with  figure  28,  and  also  represents  the  ordinary 
form  from  the  Niagara  Group  of  Indiana  and  New  York. 
Each  locality,  however,  presents  minor  differences,  mainly 
of  local  interest,  and  seldom  of  varietal  importance.  In 
western  New  York,  besides  the  ordinary  form  with  both 
valves  convex,  there  is  found  an  arcuate,  deeply  bilobed 
variety,  agreeing  with  the  extreme  of  the  Swedish  B.  bilobus, 
var.  Verneuilianus  Lindstrom,  represented  in  Plate  XXIII, 
figure  2.  The  lobes  of  the  New  York  variety  are  commonly 
more  divergent,  as  shown  in  the  outline,  Plate  XXIII, 
figure  1.  This  form  was  recently  described  by  Ringueberg 
as  Orthis  acutiloba.* 

The  Lower  Helderberg  species  known  as  B.  various  Conrad, 
sp.,  presents  an  amount  of  departure  from  typical  B.  bilobus, 
as  would  be  anticipated  from  the  change  in  the  chronological 
and  physical  conditions  of  the  species,  combined  with  its 
extremely  prolific  development  at  this  time.  The  abundance 
and  comparatively  large  size  of  individuals  clearly  indicate 
most  favorable  conditions  for  their  existence  and  multiplica- 
tion, and,  also,  for  the  assumption  and  transmission  of  any 
varietal  forms  in  harmony  with  the  environment. 

Mature  individuals  from  Dudle}r,  England,  and  Gotland, 
Sweden,  represented  by  figure  28,  Plate  XXIII,  correspond 
in  all  characters  with  specimens  of  B.  various  which  are 
about  half  or  two-thirds  grown.  After  reaching  the  adult 
bilobus  stage,  B.  various  continues  its  growth,  but  this  sub- 
sequent increment  is  gerontic  in  its  nature,  although  such 
senile  features  are  here  the  conditions  of  simple  maturity  or 
the  completed  ephebic  stage.  Evidences  of  this  are  seen  in 
the  gradual  obsolescence  of  the  pronounced  lobation  of  the 

*  Proc.  Acad.  Nat.  Sci.  Phila.,  134,  1888. 


DEVELOPMENT  OF  BILOBITES  401 

shell  and  the  cessation  of  areal  growth  in  the  neanic  period. 
The  form  known  as  B.  bilobus^  var.  Verneuilianus^  Liiid- 
strb'm,  from  Gotland,  shows  a  tendency  to  develop  in  the 
opposite  direction,  as  the  lobation  becomes  more  and  more 
pronounced  with  growth,  and  the  shell  exceeds  in  size  the 
normal  species.  The  decrease  in  the  lobation  of  B.  various 
is  a  degeneration  towards  an  embryonic  character,  while  the 
arrested  areal  development  produces  a  condition  of  partial 
isomorphism  resembling  one  of  the  higher  groups  of  Orthis, 
such  as  RTiipidomella  (R.  Michelini  L'Eveille). 

From  what  has  been  stated,  it  seems  evident  that  the  form 
typified  by  B.  bilobus  from  the  Niagara  was,  at  that  time, 
not  a  very  plastic  type,  and  capable  of  only  slight  degrees  of 
variation  or  departure  from  the  normal  form.  Naturally,  all 
the  modifications  which  occur  containing  a  differentiation  of 
the  essential  idea  of  the  genus  appear  in  the  early  history 
of  the  group,  and  are  found  previous  to  the  Lower  Helder- 
berg  form.  The  latter  species  while  losing,  in  a  manner,  its 
bilobus  expression  at  maturity,  .degenerates  into  forms  resem- 
bling ancestral  and  other  groups. 

The  material  for  the  basis  of  this  paper  was  collected  by 
the  writer  from  the  lower  members  of  the  Shaly  Limestone 
of  the  Lower  Helderberg  Group,  along  the  top  of  the  main 
escarpment  of  the  Helderberg  Mountains,  between  Clarks- 
ville  and  the  Indian  Ladder,  Albany  county,  New  York. 
Half-grown  and  fully  developed  specimens  of  Bilobites  vari- 
ous Conrad,  sp.,  can  still  be  picked  up  in  considerable 
numbers  in  the  soil  formed  of  the  decomposed  limestones. 
The  species,  however,  is  not  so  abundant  as  formerly.  Pro- 
fessor James  Hall  is  authority  for  the  statement  (Pal.  N.  Y., 
vol.  iii,  p.  493)  that  forty  thousand  individuals  were  col- 
lected between  1843  and  1853,  and  about  four  thousand  in 
the  four  following  years.  The  young  specimens  have  been 
obtained  only  by  carefully  examining  the  decomposed  sur- 
faces of  the  limestones,  and  by  treating  with  hydrochloric 
acid  slabs  of  rock  in  which  the  fossils  are  replaced  by  silica. 

26 


402  STUDIES  IN  EVOLUTION 

After  considerable  labor  and  search,  about  a  thousand  indi- 
viduals have  been  obtained.  From  this  number  it  has  been 
possible  to  select  a  series  of  over  forty  specimens,  showing 
stages  of  growth  ranging  from  shells  a  little  less  than  one- 
half  a  millimetre  in  length  to  a  length  of  nine  millimetres, 
thus  representing  the  development  between  these  limits  by 
almost  insensible  gradations. 

Developmental  Changes  in  BiloUtes  various. 

In  the  youngest  specimens  yet  detected,  measuring  .49 
mm.  in  length,  and  semi-elliptical  in  outline,  the  dorsal 
valve  is  longer  than  the  ventral;  the  hinge  is  equal  to  the 
greatest  width  of  the  shell;  both  areas  are  high,  sub-equal, 
and  perforated  by  a  triangular  fissure  in  each  valve.  In  rare 
instances  the  pedicle  covering  or  pseudo-deltidium  is  appar- 
ently retained  in  young  shells. 
Figure  131  of  the  ventral  area 
shows  the  fissure  and  pedicle  cov- 
ering, with  the  foramen  at  the 
apex  of  the  beak.  The  covering 
is  soon  absorbed  or  abraded  dur- 

FIGURE  131.—  Bilobites  vari-      .  ,  .,  ,    jn 

cus,  Conrad;  ventral  area.    X  25.     ing    Subsequent    growth,    and    the 

pedicle  then  emerged  through  the 

fissure  below.  None  of  these  characters  obtain  in  the  neanic 
or  ephebic  stages,  which  are  represented  by  a  cordate,  bilobed 
shell ;  dorsal  valve  shorter  than  the  ventral ;  hinge-line  much 
shorter  than  the  width  of  the  shell,  and  an  inconspicuous 
dorsal  area  without  a  fissure. 

The  series  of  outlines  (Plate  XXIII,  figures  11  to  26) 
drawn  to  the  same  scale,  illustrate  both  the  important 
changes  which  take  place  in  the  general  form,  and  the  corre- 
sponding increase  in  size  from  stage  to  stage.  The  rounded 
frontal  margin  of  figures  11  and  12  becomes  straight  in  figure 
13,  and  in  figure  14  a  gentle  sinus  is  apparent,  which  is  pro- 
nounced in  figure  15,  and  thereafter  is  the  conspicuous  char- 
acter of  the  entire  shell  up  to  the  ephebic  stage  represented 
by  figure  23.  Figures  24  and  25  show  that  upon  reaching 


DEVELOPMENT  OF  BILOBITES  403 

maturity  a  gerontic  tendency  to  obliterate  the  marginal  sinus 
is  initiated;  thus  degenerating  to  an  embryonal  condition  of 
lobation  similar  to  figure  14. 

The  length  of  the  hinge-line  from  an  initial  dimension 
equal  to  the  greatest  width  of  the  shell  becomes  equal  to  but 
one-half  the  width  of  the  shell  in  a  specimen  3.5  mm.  wide; 
and  in  a  full-grown  individual,  as  represented  by  figure  25, 
the  hinge  is  not  more  than  one-quarter  the  width  of  the  shell. 
From  having  sub-equal  areas  the  change  is  rapid,  so  that  in 
a  very  early  stage,  but  two  or  three  removes  from  the  initial 
one  of  the  series,  the  ventral  area  is  the  larger  and  the  fissure 
higher.  This  ratio  progressively  increases,  and  after  the 
shell  reaches  a  length  of  1.5  mm.,  the  dorsal  area  ceases  to 
be  a  conspicuous  feature.  All  areal  growth  and  hinge  exten- 
sion end  in  the  middle  neanic  period,  and  to  this  cause  is  due 
the  great  disparity  between  the  length  of  the  hinge  and  the 
width  of  the  shell  in  ephebic  individuals.  The  nepionic 
shells  show  some  extension  of  the  cardinal  angles,  but  the 
auriculation  does  not  become  apparent  until  the  lobation  of 
the  valves  is  initiated. 

On  account  of  the  greater  length  of  the  incipient  dorsal 
valve  and  consequent  obliquity  of  the  area,  the  fissure  and 
area  of  that  valve  may  be  seen  when  the  shell  is  viewed  from 
the  ventral  side,  as  in  figure  10,  and,  consequently,  the  ven- 
tral area  is  concealed  from  the  dorsal  aspect,  as  shown  in 
figures  3-9  and  11-15.  This  is  a  remarkable  reversion  of 
characters,  and  one  which  appears  to  be  of  considerable 
significance  from  a  phylogenetic  standpoint. 

The  radiating  striae  first  appear  on  the  lower  half  of  the 
initial  shell  of  the  series,  indicating  that  in  an  earlier  condi- 
tion the  shell  was  smooth.  The  striae  appear  in  pairs.  The 
first  two  striae  extend  to  the  antero-lateral  borders.  An 
additional  intercalated  pair  is  next  introduced,  together  with 
a  single  one  on  each  side  between  the  primary  radii  and  the 
cardinal  border.  The  number  after  this  stage  is  more  rapidly 
increased  by  increment  in  the  cardinal  lateral  areas  than  in 
the  median  region. 


404 


STUDIES  IN  EVOLUTION 


Observations.  —  As  shown  in  the  ontogeny  of  B.  various., 
the  generic  stock  was  derived  from  a  radicle  having,  in  many 
respects,  the  characters  of  the  group  represented  by  Platy- 


132 


<u 

I 


•'b 


.2 

f 


O 


FIGURE  132.  —  Genesis  of  Bilobites. 

a,  nepionic  stage.     Ordovician  type  like  Platystrophia  biforata.     X  4. 

b,  nealogic  period  at  which  divergence  begins.     X  4. 

c,  Bilobites  bilobus.    Niagara  Horizon.     X  2. 

d,  Bilobites  Verneuilianus,  Niagara  Horizon.     X  2. 

e,  Bilobites  various.    Lower  Helderberg  Horizon.     X  2. 

strophia  biforata.  The  general  proportions  of  the  nepionic 
shell  in  B.  various  resemble  it  very  closely.  The  length  of 
the  hinge  at  this  period,  the  high  hinge  areas  in  both  valves, 


DEVELOPMENT  OF  BILOBITES  405 

with  sub-equal  triangular  fissures,  and  the  extent  of  the 
dorsal  and  ventral  beaks,  are  characters  very  much  the  same 
as  in  Platystrophia  biforata. 

These  features  are  maintained  until  the  neanic  stage  repre- 
sented by  figure  15,  after  which  arrested  hinge  extension  and 
increasing  areal  growth  in  the  ventral  valve  rapidly  oblit- 
erate the  early  characters,  and  in  addition  the  growing  loba- 
tion  of  the  valves  emphasizes  the  expression  of  Bilobites. 

The  genesis  of  the  species  is  represented  in  figure  132,  in 
which  it  is  shown  that  all  these  species  are  alike  in  their 
development  up  to  an  early  neanic  period  (b).  B.  Verneuilia- 
nus  (d)  diverges  at  this  point,  progressively  increasing  its 
variation  from  the  normal  direct  growth,  as  exemplified  in 
B.  bilobus  (c).  B.  various  (e)  passes  through  all  the  bilobus 
stages,  and  culminates  in  larger  individuals,  with  less  pro- 
nounced lobation  of  the  shell. 

The  direct  line  of  development  is  represented  by  B.  bilobus, 
and  it  is  significant  that  this  form  also  has  the  greatest  geo- 
logical and  geographical  distribution.  Next,  the  divergent 
and  in  direct  line,  typified  by  B.  Verneuilianus  and  B.  acuti- 
lobus,  is  also  widely  distributed,  but  less  so  than  the  first. 
Finally,  the  gerontic  form,  B.  varicus,  culminated  and  dis- 
appeared within  very  narrow  time  and  regional  limits. 


6.    DEVELOPMENT   OF   TEREBRATALIA 
OBSOLETA   DALL* 

(PLATES  XXIV  and  XXV) 

FISCHER  and  (Ehlertf  have  given  a  full  account  of  the 
development  of  the  brachial  supports  in  TerebrateUa  dorsata 
and  Magellania  venosa,  from  Tierra  del  Fuego.  This  work, 
together  with  that  of  Friele,J  and  Deslongchamps,§  on  the 
northern  species  Macandrevia  cranium  and  Dallina  septigera, 
and  the  equatorial  species  Muhlfeldtia  sanguined,  constitute 
nearly  all  that  is  known  regarding  the  metamorphoses  taking 
place  in  the  brachial  supports  during  the  growth  of  an  indi- 
vidual belonging  to  the  higher  genera  of  the  Terebratellidse. 

It  is  of  interest  to  add  another  species  to  this  list,  espe- 
cially as  it  represents  a  northern  form,  the  development  of 
which  has  not  been  hitherto  studied.  This  form  offers,  more- 
over, some  additional  features  for  comparison,  and  two  very 
early  stages  have  been  discovered  which  are  both  of  genetic 
value. 

The  material  for  this  work  has  been  kindly  furnished  by 
Dr.  William  H.  Dall,  of  Washington.  The  specimens  were 
dredged  by  the  U.  S.  steamer  Albatross,  in  113  fathoms,  at 
Station  2984,  off  Cerros  Island,  Lower  California.  In  a 
report  on  some  shells  from  this  expedition,  by  Dr.  Dall,  || 
this  brachiopod  was  described  as  TerebrateUa  occidental^ 
var.  obsoleta.  Subsequent  study,  however,  has  led  him  to 

*  Trans.  Conn.  Acad.  Sci.,  IX,  392-395,  398,  399,  pis.  ii,  iii,  1893. 

t  Bull.  Soc.  Hist.  Nat.  d'Autun,  V,  5  plates,  1892. 

}  Arch.  Math.  Nat.,  Bd.  XXIII,  1877. 

§  Etudes  critiques  des  Brachiopodes  nouveaux  ou  peu  connus,  1884. 

||  Proc.  U.  S.  Nat.  Mus.,  XIV,  1891. 


DEVELOPMENT  OF  TEREBRATALIA    OBSOLETA         407 

consider  it  as  a  distinct  species,  T.  obsoleta  (Plate  XXIV, 
figures  6-9),  and  this  determination  is  here  adopted.  A 
comparison  of  the  two  forms  shows  that  T.  occidentalis  is 
much  wider  and  less  convex,  the  plications  stronger,  and  the 
valves  heavier.  In  the  previous  paper  it  was  stated  that 
Terebratalia  obsoleta  was  morphically  equivalent  to  Terebra- 
tella  dorsata,  and  that,  during  growth,  both  went  through 
different  series  of  transformations  to  reach  this  final  equiva- 
lent type  of  structure.  It  is  now  proposed  to  illustrate  and 
describe  in  greater  detail  these  metamorphoses  of  T.  obsoleta. 
The  generic  homologies  and  differences  are  discussed  in  the 
paper  already  mentioned. 

The  earliest  stage  observed  in  this  species  (Plate  XXIV, 
figure  10)  has  a  length  of  .3  mm.  It  is  comparable  to  an 
early  stage  of  Cistella  neapolitana  described  by  Kovalevski.* 
Both  agree  in  having  an  incomplete  circlet  of  centripetal 
tentacles.  The  smaller  and  younger  tentacles  are  near  the 
front  margin,  while  those  first  formed  are  on  each  side  of 
the  mouth.  This  condition  agrees  with  the  tentacular  multi- 
plication described  by  Brooks  in  G-lottidia,  by  Kovalevski 
in  Cistella  and  Lacazella,  and  by  Morse  in  Terebratulina. 
Tentacles  £1,  ti'  were  the  first  pair  to  be  formed,  and  te, 
f2',  £3,  £3',  M,  t\'  followed  in  pairs  in  the  order  named;  f5 
has  just  appeared,  and  the  corresponding  one  on  the  other 
side  is  not  yet  seen.  Figure  10  also  shows  the  tooth-like 
projections  (cZs),  forming,  with  the  cardinal  angles,  sockets  for 
the  reception  of  the  teeth  in  the  ventral  valve.  The  adductor 
muscles  are  indicated  at  at?,  and  the  diductors  at  did;  the 
mouth  is  at  w,  with  the  visceral  mass  posterior  to  it.  No 
diverticula  have  yet  appeared  to  form  the  pallial  sinuses. 

When  the  shell  has  reached  a  length  of  .65  mm.  (Plate 
XXIV,  figure  11),  the  circlet  of  tentacles  is  complete,  and 
the  pallial  sinuses  appear  as  two  slightly  branched  tubes 
extending  anteriorly  from  the  sides  of  the  visceral  mass. 
From  the  correspondence  of  the  structure  of  Gwynia  to  this 

*  See  C.  E.  Beecher,  Development  of  the  Brachiopoda,  pt.  ii,  figs.  15,  16. 
Amer.  Jour.  Sci.t  XLIV,  1892. 


408  STUDIES  IN  EVOLUTION 

stage  of  Terebratalia  and  other  genera  of  the  Terebratellidse 
it  is  called  the  gwyniform  stage. 

By  the  time  a  length  of  1  mm.  is  attained  (Plate  XXIV, 
figure  12,  and  Plate  XXV,  figure  1)  a  triangular  septum  is 
visible  anterior  to  the  middle  of  the  dorsal  valve.  Its  eleva- 
tion inflects  the  circlet  of  tentacles,  making  the  lophophore 
reniform  or  bilobed,  and  producing  a  brachial  structure 
similar  to  that  in  Cistella.  This  stage  is  therefore  called 
the  cistelliform  stage.  The  only  advance  over  this  condition 
shown  in  adult  Cistella  is  that  the  band  supporting  the  cirri 
is  calcified  and  attached  to  the  crural  points.  In  Terebra- 
talia, during  the  growth  immediately  following  and  still 
included  in  the  cistelliform  stage,  the  septum  increases  in 
height,  and  often  two  small  lateral  expansions  appear  on  the 
posterior  sloping  edge  (Plate  XXV,  figures  3,  4). 

In  the    transformation  to  the  next,  or  platidiform,  stage 
(figures  5-8),  the  crural  points  appear,  and  there  is  a  narrow 
groove  along  the  top  of  the  septum,  which  is  arched  over  at 
the  posterior  end,  forming  at  this  point  a  small    cylinder, 
nearly  vertical  to  the  floor  of  the  valve.     This  is  the  begin- 
ning  of   the   ascending   branches,  or   secondary  loop.     The 
septum   is    quadrangular,  and  develops  on  its  anterior  edge 
several  spinous  processes  (figure  7).     The  growth  of  the  de- 
scending branches  from  the  crura  and  their  union  with  the 
septum  bring  about  the  platidiform  stage,  as  represented  in 
Plate   XXV,   figure   8;   and    the   growth    of   the  ascending 
branches  produces  the  ismeniform  structure  (figure  9).     The 
expansions  on  the  sides  of  the  secondary  loop  are  not  present 
in  all  specimens,  and  do  not  seem  to  be  important  characters. 
The  cirrated  band  at  this  time  (Plate  XXIV,  figures  4,  5) 
extends  continuously  from  behind  the  mouth  to  the  origin 
of  the  descending  branches,  and  along  them  to  the  septum, 
thence   obliquely  backward   on   the   septum,    and   over  the 
ascending  branches  to  the  median  line,  where  new  cirri  are 
introduced. 

The  ascending  lamellae  from  the  septum  already  have  begun 
to  divide  or  separate  anteriorly,  and  in  the  next  stage  (Plate 


DEVELOPMENT  OF  TEREBRATALIA    OBSOLETA        409 

XXV,  figure  10)  they  show  lacunae  produced  by  resorption. 
The  structure  of  the  loop  at  this  time  agrees  with  that  in 
Muhlfeldtia,  and  this  period  of  development  has  been  called 
the  muhlfeldtiform  stage. 

The  ends  of  the  descending  branches  have  continued  to 
widen  on  the  septum  (Plate  XXV,  figure  11),  and  extend 
toward  the  ascending  branches,  with  which  they  soon  join 
(figure  12),  bringing  in  the  terebrataliform  type  of  structure. 
The  completion  of  this  stage  is  accomplished  by  the  further 
separation  of  the  ends  of  the  ascending  branches,  and  by  the 
resorption  of  the  expanded  ends  of  the  descending  branches 
to  form  the  connecting  bands  with  the  septum  (Plate  XXV, 
figures  13,  14,  15). 

It  should  be  noted  that  the  septum  in  the  cistelliform  stage 
is  wholly  anterior  to  the  middle  of  the  length  of  the  dorsal 
valve.  Septal  growth  takes  place  chiefly  on  the  posterior 
end,  and  at  the  same  time  resorption  along  the  anterior  edge 
serves  to  move  the  septum  backward,  until  by  the  time  the 
terebrataliform  stage  is  reached  it  is  posterior  to  the  middle, 
and  in  adult  specimens  it  is  in  the  umbonal  region. 

A  comparison  of  the  growth-stages  of  T.  obsoleta  with 
those  in  Macandrevia  cranium  and  Dallina  septigera  shows 
considerable  similarity,  except,  of  course,  in  the  adult  con- 
dition. The  general  features  of  each  are  alike,  and  may  be 
correlated  in  the  same  manner,  stage  for  stage.  The  septum 
in  T.  obsoleta  in  the  platidiform  stage  is  considerably  broader 
than  in  the  other  forms,  and  the  descending  branches  join  it 
considerably  lower  down.  The  two  stages  preceding  the 
platidiform,  which  present  the  brachial  structure  first  of 
G-wynia  and  then  of  Cistella,  are  of  chief  importance. 


7.    DEVELOPMENT   OF  THE   BRACHIAL   SUP- 
PORTS  IN  DIELASMA   AND   ZYGOSPIRA* 

(PLATE  XXVI) 

It  has  been  shown  by  several  authors  f  that  the  brachial 
supports  in  the  Terebratellidse  pass  through  a  series  of  dis- 
tinct metamorphoses  during  the  life  of  the  animal.  In  the 
higher  genera  these  stages  may  be  correlated  with  the  adult 
structures  of  lower  forms,  thus  furnishing  satisfactory  data 
for  a  systematic  arrangement  of  the  genera  and  for  their 
phylogenetic  relations. 

This  kind  of  research  naturally  requires  ontogenetic  series 
of  considerable  completeness,  and  it  is  often  difficult  or  im- 
possible to  obtain  such  material  representing  fossil  forms. 
Moreover,  the  fossils  must  be  exceptionally  well  preserved  to 
afford  a  means  of  working  out  the  development  of  a  struc- 
ture so  delicate  as  the  calcareous  lamellae  supporting  the 
brachia,  especially  in  young  specimens  from  one  to  five 
millimetres  in  length. 

It  first  seemed  desirable  to  determine  the  development  in 
some  genus  of  the  Terebratulidse  from  the  Paleozoic,  in  order 
to  ascertain  whether  the  brachial  supports,  as  in  Neozoic  and 
recent  forms,  passed  through  a  series  of  transformations,  and 
to  determine  the  most  primitive  form  of  the  loop  in  the 
Ancylobrachia.  For  this  purpose  a  species  of  Dielasma 
(D.  turgidum)  obtained  from  Mr.  Moritz  Fischer  was  used. 
The  specimens  are  from  the  St.  Louis  Group  of  the  Lower 
Carboniferous  in  Kentucky.  The  shells  are  partially  silici- 

*  Beecher  and  Schuchert.  Proc.  BioL  Soc.  Washington,  VIII,  71-78,  pi.  x, 
1893. 

t  Davidson,  Friele,  Deslongchamps,  Fischer  and  CEhlert,  and  Beecher. 


BRACHIAL  SUPPORTS  IN DIELASMA  AND  ZYGOSPIRA  411 

fied,  generally  filled  with  transparent  calcite,  and  afford  very 
satisfactory  preparations  of  the  arm  supports.  It  was  found 
that  the  loop  of  Dielasma  underwent  transformations  during 
growth,  and  that  the  earliest  stage  observed  is  like  JCentro- 
nella.  This  establishes  the  centronelliform  loop  as  the  sim- 
plest type  of  loop  in  the  Ancylobrachia.  Besides  Centronella, 
other  adult  representatives  of  the  same  structure  are  Rens- 
selceria  and  Newberria.  They  are  all  late  Silurian,  Devonian, 
and  Carboniferous  genera,  but  the  centronelliform  structure 
continues  later,  and  is  represented  in  the  Trias  by  the  genera 
Juvavella  Bittner  and  Nudeatula  (Zugmayer)  Bittner. 

It  was  at  once  suggested  that  interesting  results  might  be 
obtained  in  studying  the  development  of  a  spire-bearing 
brachiopod,  and,  as  the  earliest  species  more  clearly  show 
their  phylogeny  in  their  ontogeny,  the  ancient  genus  Zygos- 
pira  was  selected.  Very  complete  material  was  accessible, 
collected  by  the  writers,  from  the  Trenton  of  Minnesota  and 
Kentucky,  so  that  series  of  specimens  were  assembled  repre- 
senting all  stages  of  growth  from  specimens  .8  mm.  in  length 
to  mature  size.  They  were  prepared  to  show  their  brachial 
supports,  and  it  is  clearly  demonstrated  that  the  primitive 
arm  support  in  Zygospira  is  a  terebratuloid  loop  having  a 
Centronella-like  form,  which  undergoes  several  modifications 
before  the  growth  of  the  spiral  lamellae,  thus  in  so  far  re- 
sembling the  development  of  Dielasma. 

These  results  threw  doubt  on  a  number  of  Lower  and 
Upper  Silurian  species  described  as  having  recurved  loops, 
and  previously  referred  to  the  higher  terebratuloid  genera 
Macandrevia  or  Waldheimia.  The  shells  are  impunctate, 
while  Rensselceria  and  Centronella  are  distinctly  punctate, 
like  all  other  well-known  Terebratulse.  Upon  investigation 
it  has  been  ascertained  by  Hall  and  Clarke  and  the  writers 
that  the  species  which  have  been  referred  to  Hallina  and 
Macandrevia  from  the  Silurian  are  spire-bearing  forms,  and 
therefore  do  not  belong  to  the  Ancylobrachia. 

Fischer  and  QEhlert  have  called  attention  to  a  number  of 
recent  species  which  have  been  erroneously  based  upon  the 


412  STUDIES  IN  EVOLUTION 

immature  stages  of  higher  species,  and  in  the  Terebratellidse 
it  is  evident  that  great  uncertainty  must  exist  in  the  identi- 
fication of  specimens  not  fully  adult.  Now,  finding  that 
Paleozoic  genera  of  both  loop  and  spire-bearing  stocks  (Ancy- 
lobrachia  and  Helicopegmata)  in  the  adolescent  period  like- 
wise pass  through  metamorphoses  representing  the  structures 
of  other  genera  and  even  other  sub-orders,  it  is  manifest  that 
species  cannot  be  referred  to  their  proper  genera,  nor  genera 
correctly  defined,  unless  the  individuals  studied  are  adult  and 
their  characters  constant  for  a  definite  period  of  time. 

Development  of  the  Loop  in  Dielasma  turgidum. 

The  earliest  stage  thus  far  observed  was  found  in  a  speci- 
men a  little  over  four  millimetres  in  length  (Plate  XXVI, 
figure  1).  The  loop  at  this  time  is  composed  of  two  broad 
descending  lamellsB,  which  begin  at  the  ends  of  the  crura  and 
extend  forward,  curving  ventrally  until  they  unite  in  the 
median  line,  forming  an  angular  ridge,  acuminate  in  front. 
As  previously  mentioned,  this  structure  is  very  similar  to 
that  of  Centronella,  and  this  stage  is  therefore  called  the 
centronelliform  stage. 

The  first  change  in  the  form  of  the  loop  is  brought  about 
by  a  resorption  of  the  pointed  anterior  portion,  so  that  the 
outline  is  re-entrant  in  front  (figure  2).  Further  resorption 
in  the  same  manner  results  in  the  production  of  two  poste- 
riorly directed  branches,  as  shown  in  figure  3.  This  form 
may  be  considered  as  an  early  immature  Dielasma  loop,  as 
subsequent  growth  does  not  materially  modify  its  general 
characters. 

The  adult  loop,  represented  in  figures  4-6,  differs  from 
the  early  Dielasma  stage  chiefly  in  the  divergence  of  the 
descending  branches. 

In  the  centronelliform  stage  the  lamellae  converge,  and  the 
loop  extends  half  the  length  of  the  shell.  Both  of  these 
relations  gradually  alter  until,  in  the  early  Dielasma  stage, 
the  descending  branches  are  nearly  parallel,  the  loop  extends 
less  than  half  the  length,  and  finally,  when  mature,  the 


BRACHIAL  SUPPORTS  IN DIELASMA  AND  ZYGOSPIRA  413 

descending  branches  diverge  and  the  loop  is  two-fifths  the 
length  of  the  dorsal  valve. 

The  natural  inferences  to  be  drawn  from  the  development 
of  the  loop  in  Dielasma  are,  that  Oentronella  represents  a 
larval  or  immature  condition  of  the  higher  genera,  and  that 
the  centronelloid  loop  is  the  primitive  type  in  the  Terebra- 
tulidse.  Therefore,  as  Centronella  and  the  closely  related 
genus  Rensselceria  are  the  only  early  punctate  terebratuloids 
known,  and  as  they  have  the  primitive  type  of  loop,  there 
arises  the  question  of  the  validity  of  the  Upper  and  Lower 
Silurian  species  with  recurved  loops,  referred  to  Waldheimia 
and  Hallina. 

Hall  and  Clarke  (Pal.  N.  F.,  vol.  viii,  part  ii,  pp.  147- 
153,  not  yet  published)  describe  and  figure  the  brachial  sup- 
ports in  Hallina,  showing  that  both  H.  Nicoletti  Winchell 
and  Schuchert  and  H.  Saffordi  Winchell  and  Schuchert  are 
provided  with  short  spires  of  about  one  volution,  connected 
by  a  transverse  band,  as  in  Zygospira.  In  removing  the  ven- 
tral valve  and  exposing  the  loop  from  that  side,  as  is  often 
done,  the  short  spiral  lamellae  have  been  overlooked.  Similar 
observations  have  been  made  by  the  present  writers,  so  that 
the  systematic  position  of  these  forms  is  now  established. 

Specimens  of  Waldheimia  bicarinata  Angelin,  from  the 
Upper  Silurian  of  Gotland,  were  also  examined.  They  were 
found  to  possess  well-defined  spiral  cones,  and  in  other  re- 
spects agreed  with  the  diagnosis  of  Dayia.  These  facts 
indicate  that  the  specimens  described  by  Davidson  as  Wald- 
heimia Maivii  (Fossil  Brachiopoda,  Supp.  vol.  iv,  pt.  v,  pi. 
iv,  figs.  1-3)  are  the  young  of  Dayia  navicula  Sowerby,  sp. 
(ibid.,  pi.  v,  figs.  1-4). 


Development  of  the  Brachial  Supports  in  Zygospira 
recurvirostris. 

The  smallest  specimen  in  which  the  internal  structure  was 
observed  measures  about  1.33  mm.  in  length  (Plate  XXVI, 
figures  7,  8).  The  brachial  supports  consist  of  two  straight, 


414  STUDIES  IN  EVOLUTION 

ventrally  concave,  primary  lamellae,  rapidly  increasing  in 
width  from  the  thin  crural  plates  to  near  the  centre  of  the 
valve,  where  they  unite,  forming  a  plate  with  a  central 
angular  ridge.  The  anterior  end  of  the  plate  is  pointed,  as  in 
Centronella. 

In  a  specimen  about  2  mm.  in  length  (figures  9,  10),  the 
primary  lamellae  are  practically  of  the  same  form  as  in  the 
preceding,  but  much  of  the  original  central  portion  of  the  loop 
has  been  resorbed,  so  that  the  lamellae  are  connected  by  a 
short  but  comparatively  wide,  ventrally  arched,  transverse 
band.  The  lamellae,  or  descending  branches,  are  also  more 
spreading  anteriorly,  and  there  is  a  slight  deflection  at  the 
crural  points  which  becomes  more  and  more  pronounced  as 
growth  progresses. 

In  the  next  stage  (figure  11),  which  has  a  length  of  2.33 
mm.,  the  descending  branches  are  more  diverging,  and  the 
transverse  band  is  longer  and  more  broadly  excavated  in 
front. 

The  succeeding  stages  here  described  are  based  upon  mate- 
rial derived  from  near  the  top  of  the  Trenton,  where  the 
specimens  of  this  species  are  usually  larger  and  more  trans- 
verse than  those  from  near  the  base  of  the  Trenton,  which 
is  the  horizon  of  the  specimens  illustrated  in  figures  7-11. 
Therefore,  when  the  loop  in  figure  9  is  compared  with  that 
of  figure  12,  it  is  seen  that  the  latter  is  much  the  wider,  from 
the  greater  size  and  breadth  of  the  shell,  which  has  at  this 
stage  a  length  of  3.33  mm.,  while  the  former  is  but  2  mm. 
long.  The  loop  in  figure  12  is  somewhat  more  advanced 
than  in  figure  9,  the  transverse  band  being  narrower  and 
slightly  elevated  posteriorly,  some  resorption  having  taken 
place  along  the  inner  edges  of  the  primary  lamellae.  Further 
resorption  in  same  direction  produces  the  brachial  support 
illustrated  in  figure  14.  This  form  of  loop  in  Z.  Nicoletti, 
Z.  Saffordi,  and  Z.  recurvirostris  from  the  lowest  Trenton  is 
retained  to  maturity.  However,  in  specimens  of  Z.  recurvi- 
rostris from  the  upper  Trenton  the  posteriorly  curved,  trans- 
verse band  is  not  a  mature  feature,  since  it  becomes  changed 


BRACHIAL  SUPPORTS  IN  DIELASMA  AND  ZYGOSPIRA  415 

into  the  form  represented  in  figure  15.  In  previous  stages 
the  transverse  band  is  ventrally  arched,  but  it  now  bends 
dorsally,  and  remains  so  during  subsequent  growth  until  near 
maturity,  when  the  sinus  of  the  dorsal  valve  causes,  it  to 
assume  a  sigmoid  curve. 

The  spirals  next  begin  to  develop  (figures  16  and  17)  as 
two  slender  converging  lamellae,  curving  toward  the  ventral 
valve  and  originating  from  the  outer  pointed  ends  of  the 
loop.  These  lamellae  then  incurve  dorsally  and  laterally  to  a 
point  just  posterior  to  the  transverse  band,  forming  the  first 
volution  of  a  spiral  (figure  18).  In  this  manner  further 
growth  and  elongation  of  the  lamellae  continue  until  maturity 
is  attained,  when  there  are  about  three  volutions  in  each 
spiral  cone  (figure  20).  The  calcareous  brachial  supports 
occupy  about  the  same  relative  space  in  the  shell  cavity  in  all 
stages  of  growth. 

Observations  and  Correlations. 

Zygospira  is  the  earliest  spire-bearing  genus  known,  as  it 
is  found  in  the  Birdseye  Limestone  of  the  Trenton  period. 
It  is  of  considerable  interest,  therefore,  to  study  the  develop- 
ment of  the  spirals.  From  the  ontogeny,  it  is  shown  that 
the  brachial  supports  in  Zygospira  begin  as  a  loop  greatly 
resembling  that  of  Devonian  Oentronella.  Moreover,  the 
loop  passes  through  a  series  of  metamorphoses  before  the 
spirals  make  their  appearance. 

The  most  ancient  species  are  Z.  Nicoletti  and  Z.  Saffordi, 
small  semi- plicate  forms,  in  which  the  spirals  are  very  rudi- 
mentary, consisting  of  about  one  volution.  In  the  same 
geological  horizon  occurs  Z.  recurvirostris,  having  from  two 
to  two  and  one -half  turns  of  the  lamellae  in  each  spiral. 
The  same  species  from  the  upper  Trenton  has  three  volu- 
tions, while  in  Z.  modesta  of  the  middle  Lorraine  there  are 
from  four  to  five  whorls  (figure  25).  In  Z.  Headi  (figure 
24)  a  large  globose  finely  striated  species  of  the  upper  Lor- 
raine, there  are  six  whorls  to  a  cone.  The  geological  his- 


416  STUDIES  IN  EVOLUTION 

tory,  therefore,  shows  a  gradual  increase  of  from  one  to  six 
turns  of  the  lamellae  in  each  spiral. 

The  transverse  band  connecting  the  primary  lamellae  also 
undergoes  a  series  of  changes.  It  has  been  shown  that  the 
centronelloid  loop  (figure  7)  passes  into  one  having  the 
lamellae  joined  by  a  posteriorly  directed,  transverse  band 
(figure  14).  This  form  of  loop  is  retained  as  a  mature 
feature  in  the  brachia  of  Z.  Nicoletti,  Z.  Saffordi,  and  in  the 
lower  Trenton  varieties  of  Z.  recurvirostris.  Passing  to  the 
specimens  of  the  latter  species,  which  are  geologically  later, 
the  band  no  longer  joins  the  lamellae  as  far  anteriorly  as  in 
the  older  variety  (figure  20).  The  point  of  connection  in 
Z.  modesta  is  variable  (figures  25  and  26),  but  is  usually 
more  posterior  than  in  Z.  recurvirostris,  while  in  Z.  Headi  it 
is  manifestly  more  posterior  than  in  any  of  the  older  species 
of  Zygospira.  The  transverse  band  is  now  no  longer  arched 
backward,  but  is  just  the  reverse  (figure  24),  while  its  posi- 
tion is  progressively  more  and  more  posterior,  and  the  loop  is 
gradually  shortened  before  the  spirals  make  their  appearance. 
The  gradual  increase  in  the  number  of  the  whorls  in  each 
spiral  and  the  recession  of  the  transverse  band  have  gone  on 
together.* 

The  family  Atrypidae  includes  the  genera  Zygospira,  Glassia, 
Atrypa,  and  Dayia.  It  is  easily  distinguished  from  all  other 
families  comprised  in  the  sub-order  Helicopegmata,  since  the 
spirals  are  between  the  first  descending  branches  of  the 
lamellae,  while  in  the  Spiriferidae,  Nucleospiridae,  and  Athy- 
ridae  the  primary  lamellae  are  between  the  spirals. 

The  gradual  increase  in  the  number  of  whorls  in  the  spirals 
and  the  pushing  backward  of  the  transverse  band  in  the 
Atrypidae  is  carried  farthest  in  the  species  of  Atrypa.  In 
Coelospira  Barrandei  and  0.  marginalis  the  brachial  supports, 
as  worked  out  by  Davidson  and  Glass,  consist  of  about  five 
volutions,  and  are  similar  to  those  of  Zygospira,  except  that 

*  The  extreme  anterior  position  of  the  transverse  band  in  Z.  recurvirostris 
is  therefore  of  no  more  than  specific  value,  and  on  this  account  Anazyga  David-* 
son  cannot  well  be  separated  from  Zygospira. 


BRACHIAL  SUPPORTS  IN  DIELASMA  AND  ZYGOSPIRA   417 

the  transverse  band  is  more  posterior,  since  it  originates  near 
the  ends  of  the  crura.  This  mature  condition  of  Coelospira 
is  seen  to  be  a  young  condition  of  Atrypa  (Davidson),  but, 
as  the  spirals  are  more  loosely  coiled  and  the  transverse  band 
always  continuous,  this  genus  should  be  regarded  as  valid  in 
the  evolution  from  Zygospira  to  Atrypa.  In  mature  Atrypa 
reticularis  from  the  Upper  Silurian,  there  may  be  as  many  as 
sixteen  volutions  in  each  spiral  cone  (Davidson),  but  more 
often  the  number  is  smaller.  The  transverse  band  in  this 
species  during  its  young  stages  is  continuous,  but  in  the 
adult  condition  it  seems  to  be  usually  disunited  in  the 
middle.  This  feature  becomes  a  distinct  adult  character  in 
the  Devonian  specimens,  which  also  have  a  greater  number 
of  whorls  in  the  spirals,  as  shown  in  a  Chemung  specimen  of 
this  species  in  Yale  University  Museum,  having  twenty-four 
turns  of  the  lamellse  in  each  spiral. 

The  ontogeny  and  phylogeny  of  the  species  of  Zygospira 
indicate  strongly  that  the  Atrypidw  had  its  origin  in  a  form 
with  a  centronelloid  loop.  A  further  natural  conclusion 
from  the  same  evidence  is  that  the  Ancylobrachia  are  older 
and  more  primitive  than  the  Helicopegmata. 


27 


IV 

MISCELLANEOUS   STUDIES   IN 
DEVELOPMENT 

1.  DEVELOPMENT  OF  A  PALEOZOIC   PORIFEROUS  CORAL 

2.  SYMMETRICAL    CELL    DEVELOPMENT    IN    THE    FAVO- 

SITIDJE 

3.  DEVELOPMENT  OF  THE  SHELL  IN  THE  GENUS  TOR- 

NOCERAS  HYATT 


IV 

MISCELLANEOUS  STUDIES   IN   DEVELOPMENT 

1.     DEVELOPMENT   OF  A   PALEOZOIC 
PORIFEROUS   CORAL* 

(PLATES  XXVII-XXXI) 

THE  origin  and  affinities  of  many  groups  of  Paleozoic 
corals  are  still  obscure.  The  main  elements  of  the  recog- 
nized system  of  classification  seem  to  be  stable,  yet  so  little 
is  known  of  the  growth  and  structure  of  a  number  of  impor- 
tant groups  that  they  occupy  a  different  place  in  almost  every 
arrangement  of  the  genera.  Each  fact  of  development  affords 
data  which  eliminate,  to  a  degree,  the  want  of  knowledge 
concerning  their  origin  and  relations.  Unless  the  growth 
of  the  organism  is  obscured  by  pronounced  accelerated  or 
degradational  features,  its  interpretation  is  simple  and 
throws  much  light  on  its  ancestral  history.  Paleozoic  types 
in  general  are  least  modified  in  their  development  by  accel- 
eration. They  usually  show  some  marked  expression  of  their 
prototype,  and  also  the  succession  of  changes  through  which 
they  have  passed  during  their  evolution. 

The  species  here  discussed  was  originally  described  as 
Michelinia  lenticularis  Hall,f  from  the  Lower  Helderberg 
Group  of  New  York.  If  Michelinia  is  entitled  to  recognition, 
it  will  exclude  this  form,  as  it  is  without  tabulae.  Pleuro- 
dictyum,  as  now  defined,  is  more  in  harmony  with  these 

*  Trans.  Conn.  Acad.  Sci.,  VIII,  207-214,  pis.  ix-xiii,  1891. 
t  Twenty-sixth  Rept.  N.  Y.  State  Mus.  Nat.  Hist.,  113,  1874. 


422  STUDIES  IN  EVOLUTION 

features,  and  therefore  the  species  M.  lenticularis  is  here 
referred  to  this  genus.  The  large  calices  and  their  constant 
origin  at  the  basal  epitheca  are  not,  however,  essential 
characters  of  Pleurodictyum.  The  structure  and  growth 
of  this  species  indicate  that  it  represents  one  of  the  simpler 
types  of  poriferous  corals.  For  this  reason  its  develop- 
ment is  without  the  numerous  modifications  necessary  in 
more  complex  forms,  and  its  laws  of  growth  are  not 
complicated. 

Development  of  Pleurodictyum  lenticulare. 

The  nepionic  stage  is  well  marked.  It  comprises  the 
growth  of  the  corallum  to  the  completion  of  a  simple  initial 
cell.  This  primitive  cell,  or  nepionic  stage  (Plate  XXVII, 
figures  1,  7,  8),  has  the  form  of  an  oblique  inverted  cone, 
flattened  on  one  side.  The  flattened  area  represents  the 
lower  or  attached  side,  and  the  oblique  base  of  the  cone  is 
occupied  by  the  aperture  of  the  corallite.  The  apical  portion 
is  smooth  for  about  one-fourth  the  length  of  the  cell.  Then 
the  concentric  lines  of  growth  become  apparent,  and  over  the 
distal  half  radiating  ribs  are  also  developed.  The  interior  of 
the  apex  is  granulose.  At  about  the  middle  of  the  cell  the 
granules  are  arranged  in  rows,  forming  the  beginnings  of  the 
septal  lines. 

The  simple  growth  of  the  initial  cell  continues  until  the 
entire  procumbent  portion  is  completed.  A  thickening  of 
the  margin  then  takes  place,  and  an  upward  growth  of  the 
corallite  is  initiated.  At  the  commencement  of  this  upward 
growth  the  first  bud  starts  out  from  the  lateral  edge  of  the 
initial  calyx,  either  to  the  right  or  left  of  the  axis.  This 
condition  represents  the  first  neanic  stage.  The  bud  resem- 
bles the  parent  cell  in  all  particulars,  and  reaches  considerable 
size  before  the  second  appears,  as  shown  in  Plate  XXVII, 
figures  9,  10.  The  visceral  cavities  are  confluent,  as  the 
initial  apex  of  the  bud  opens  into  the  calyx  of  the  first  cell. 

The  succeeding  neanic  stages,  to   the  completion  of   the 


DEVELOPMENT  OF  PALEOZOIC  PORIFEROUS  CORAL    423 

first  circle  of  peripheral  calices,  have  been  observed  mainly 
from  the  epithecas  of  mature  or  nearly  full-grown  corallums, 
represented  on  Plate  XXVIII,  figures  1,  2.  In  these  ex- 
amples the  lines  of  growth  are  so  perfectly  shown  that  all 
the  stages  are  distinctly  marked,  and  may  be  satisfactorily 
studied. 

What  is  here  considered  as  the  second  neanic  stage  is 
represented  on  Plate  XXVII,  figure  3,  showing  the  initial 
corallite,  with  the  first  and  second  buds  on  opposite  sides. 
This  process  of  alternate  gemmation  from  the  parent  cell 
continues  until  the  circlet  of  calices  is  completed,  as  shown 
in  figures  4,  5,  and  6.  In  this  species  the  normal  number  of 
peripheral  calices  is  seven,  making  eight  corallites  in  the 
completed  neanic  corallum.  The  last  cells  to  be  formed  are 
(1)  the  sixth  and  seventh,  budding  from  the  anterior  side  of 
the  first  calyx,  and  (2)  the  eighth,  or  posterior  cell.  Plate 
XXVII,  figure  12,  represents  the  completed  neanic  corallum, 
with  the  initial  cell  and  six  well-developed  peripheral  calices. 
The  eighth  has  just  begun  to.  fill  up  the  space  between  the 
second  and  third.  It  will  be  noticed  that  there  is  a  direct 
correspondence  in  the  size  of  the  calices  to  their  relative 
age.  The  first  calyx  is  much  the  largest.  Then,  decreasing 
serially,  come  the  second,  third,  fourth,  fifth,  sixth,  and 
seventh,  while  the  eighth  is  undeveloped.  An  inspection 
of  the  upper  surface  of  a  mature  corallum  will  thus  usually 
determine  the  order  of  successive  calical  additions.  After 
the  appearance  of  the  posterior,  or  eighth  calyx,  the  corallum 
commonly  grows  to  double  the  diameter  of  the  completed 
neanic  stage,  resulting  in  the  normal  ephebic  or  mature  con- 
dition, as  represented  on  Plate  XXIX,  figures  1,  2.  Nearly 
all  the  full-grown  specimens  found  agree  in  this  respect. 

A  corallum  rarely  presents  any  departure  from  the  normal 
number  of  calices.  Plate  XXX,  figure  1,  is  an  example  of 
a  variation  in  the  number  of  peripheral  corallites,  for  in  this 
specimen  there  are  eight  in  the  circle  instead  of  the  usual 
seven.  A  variation  in  the  opposite  direction  is  shown  in 
another  specimen  having  five  well-developed  corallites  about 


424  STUDIES  IN  EVOLUTION 

the  parent  cell.  Old-age  characters  are  expressed  in  two 
ways:  First,  the  cell  walls  become  thickened  around  the 
margin  of  the  epitheca  without  destroying  the  symmetry  of 
the  corallum,  as  shown  in  Plate  XXX,  figure  2;  second,  by 
the  indefinite  and  unequal  development  of  the  peripheral 
cells,  together  with  the  addition  of  calices  budding  from  the 
cells  forming  the  primary  circle.  One  specimen,  appearing 
at  first  sight  as  an  example  of  cell  division  or  fission,  is 
shown  in  Plate  XXXI,  figure  1.  It  may  be  explained  as 
resulting  from  the  abnormal  growth  of  the  second  and  adja- 
cent calices,  four  and  eight.  This  lateral  impulse  further 
resulted  in  sending  off  the  small,  peripheral,  tertiary  coral- 
lites  numbered  in  the  figures  9,  10,  11,  12,  and  13. 

It  should  be  understood  that  this  arbitrary  expression  of 
normal  and  abnormal  growths  applies  only  to  the  species 
Pleurodictyum  lenticulare.  The  same  numerical  arrangement 
will  not  hold  good  for  genera  like  Favosites,  Michelinia, 
Striatopora,  etc.  Otherwise,  it  is  believed,  the  general  laws 
of  growth  here  brought  out  will  hold  good  for  these  and 
other  related  genera. 

Some  doubt  may  exist  as  to  the  propriety  of  referring  the 
specimens  illustrated  on  Plate  XXVII,  figures  9-11,  to 
P.  lenticulare.  Unfortunately,  material  of  this  kind  is  rare 
and  difficult  to  obtain.-  With  the  exception  of  the  position 
and  direction  of  the  first  bud  (figure  10),  all  the  characters 
agree,  so  far  as  can  be  observed,  with  ordinary  specimens  of 
P.  lenticulare.  The  second  cell  of  the  corallum  represented 
in  Plate  XXVIII,  figure  1,  curves  rapidly  backward,  al- 
though at  first  the  axis  has  an  anterior  direction.  Taking 
this  view  of  the  specimen  (Plate  XXVII,  figure  11),  it  is 
not  difficult  to  see  how  the  succeeding  enlargement  and 
curvature  of  the  bud  could  extend  backward,  thus  properly 
limiting  the  size  of  the  eighth  or  last  of  the  primary  circlet 
of  calices. 

The  method  of  determining  the  relative  age  and  succession 
of  the  corallites  can  be  seen  in  Plate  XXVIII,  figures  1,  2, 
and  Plate  XXIX,  figure  2.  The  initial  cell  occupies  the 


DEVELOPMENT  OF  PALEOZOIC  PORIFEROUS  CORAL    425 

central  position,  and  forms  the  boss  or  apex  of  the  basal 
epitheca.  The  first  bud  is  nearly  on  a  plane  with  the  base  of 
the  initial  cell,  and  is  the  one  nearest  the  apex.  The  second 
and  successive  buds  are  respectively  more  distant  and  at  a 
higher  level.  Specimens  having  broad  surfaces  of  attach- 
ment to  foreign  objects  have  these  distinctive  features  of  the 
epitheca  obliterated,  and  the  only  guide  to  the  order  of  the 
corallites  then  lies  in  their  comparative  size  and  position  on 
the  upper  surface  of  the  corallum. 

General  Conclusions. 

The  first  feature  to  be  noted  in  the  development  of  a 
poriferous  coral,  as  here  described,  is  the  simple  cyathiform 
character  of  the  initial  corallite.  This  nepionic  stage  is 
without  mural  pores,  and  has  an  epitheca  over  the  entire 
exterior  of  the  cup.  The  septal  lines  become  developed 
toward  the  end  of  this  stage.  These  features  are  in  harmony 
with  the  young  of  many  Paleozoic  corals,  such  as  Cladochonus, 
Aulopora,  or  Syringopora,  and  clearly  indicate  a  primitive, 
simple,  and  imperforate  ancestry  for  the  Perforata.  A  simi- 
lar origin  and  development  obtains  in  Favosites,  as  may  be 
seen  from  the  figure  of  a  young  colony  of  F.  Forlesi,  var. 
occidentalis^  given  by  Professor  Hall.* 

The  first  neanic  stage,  represented  by  the  primitive  coral- 
lite  with  one  bud,  is  the  first  transition  toward  both  a  com- 
pound and  a  perforate  coral  (Plate  XXVII,  figure  9).  This 
stage  has  two  calices  making  it  a  compound  coral,  and  has 
an  opening  through  the  cell  walls  or  connecting  channel 
between  the  corallites,  forming  the  first  mural  pore.  The 
manner  of  growth  and  the  structure  of  the  corallum  at  this 
stage  are  suggestive  of  Aulopora,  and  should  be  given  con- 
siderable significance.  The  visceral  cavities  in  Aulopora  are 
confluent,  and  rudimentary  septa  or  lines  of  spinules  are 
often  present.  Romingeria  has  a  growth  resembling  Aulo- 

*  Indiana  Geol.  and  Nat.  Hist.,  llth  Rept.  of  the  State  Geologist,  pi.  i,  fig.  12, 
1881. 


426  STUDIES  IN  EVOLUTION 

pora  and  Syringopora.  It  is  without  pores  on  the  portion 
where  the  corallites  and  buds  are  free,  but  when  these  are  in 
juxtaposition  at  their  bases  mural  pores  are  developed.  The 
upward  growth  of  the  initial  cell  of  P.  lenticulare  proceeds 
but  a  short  distance  before  the  circlet  of  peripheral  corallites 
is  completed.  Thus  at  this  stage  there  are  at  least  seven 
mural  pores  opening  into  the  primary  calyx.  If  this  ten- 
dency to  the  formation  of  numerous  buds  persists  throughout 
the  upward  growth  of  the  corallites,  the  non-development 
of  the  buds  consequent  upon  the  adjacent  living  corallites 
would  naturally  result  in  the  production  of  mural  pores. 
The  basal  epitheca  limits  the  fleshy  portion  of  the  organisms, 
and  represents  an  area  unfavorable  to  the  acquisition  of 
food  or  for  the  natural  development  of  calices.  Therefore  it 
would  prevent  both  the  maintenance  of  mural  pores  and  the 
growth  of  basal  buds.* 

A  Favosites  in  which  one  or  more  cells  became  inactive  or 
dead  shows  in  its  subsequent  growth  the  closing  over  of  this 
area  by  the  budding  of  the  surrounding  cells.  Each  cell  is 
connected  with  the  parent  by  an  apical  pore  (Plate  XXXI, 
figures  3,  4).  Without  this  opportunity  to  bud  afforded  by 
the  death  of  one  or  more  corallites,  or  by  their  divergence, 
the  adjacent  cells  would  have  developed  only  mural  pores. 
In  the  figure  of  Pleurodictyum  proUematicum  given  on  Plate 
XXXI,  figure  2,  three  of  the  initial  pores  are  indicated  by 
dotted  lines  from  p.  No  distinction  can  be  made  between 
these  and  the  ordinary  pores,  except  that  the  latter  are 
usually  not  as  large.  This  difference  in  size  would  be  ex- 
pected, as  the  primary  pore  represents  the  bud  which  suc- 

*  The  presence  of  basal  mural  pores  or  openings  through  the  epitheca  has 
heen  asserted  by  Meek  and  Worthen  (Pal.  Illinois,  III.  409,  1868).  The  spe- 
cimens from  which  this  observation  was  made,  are  from  a  friable  sandstone, 
which  does  not  usually  preserve  minute  details  with  much  distinctness.  The 
depressions  between  the  spinules  on  the  septal  lines  could  easily  be  mistaken  in 
a  cast  for  the  fillings  of  mural  pores,  and  it  is  believed  by  the  writer  that  this 
interpretation  should  be  given.  P.  lenticulare  occurs  as  calcareous  or  silicified, 
and  in  the  condition  of  casts.  No  basal  mural  pores  are  present.  Also,  none 
can  be  observed  in  the  casts  of  P.  problematicum,  from  Pelm,  Germany. 


DEVELOPMENT  OF  PALEOZOIC  PORIFEROUS  CORAL   427 

ceeded  in  producing  a  corallite ;  whereas  the  other  attempts 
at  budding  resulted  no  further  than  the  production  of  mural 
pores.  The  conclusion  to  be  drawn  is  that  the  mural  pores 
in  such  genera  as  Favosites,  Striatopora,  Pleurodictyum, 
Michelinia,  etc.,  are  ineffectual  attempts  at  budding,  result- 
ing only  in  the  perforation  of  the  cell  walls.  This  explana- 
tion agrees  with  the  pronounced  and  persistent  tendency  to 
gemmation  characteristic  of  the  genera  mentioned.  They  also 
represent  compound  forms  having  individualized  epithecas, 
and  this  feature  naturally  arises  from  the  same  system  of 
budding  obtaining  in  the  simple  corals. 

Professor  Verrill  has  shown  that  the  presence  or  absence 
of  tabulae  is  of  little  or  no  importance  in  a  natural  classifica- 
tion.* Therefore  the  non-tabulate  feature  of  P.  lenticulare 
is  without  special  consequence  in  a  discussion  of  the  rela- 
tions of  this  species  with  Favosites,  or  other  tabulate  porifer- 
ous genera. 

If  the  preceding  interpretations  of  structure  and  affinity 
are  correct,  a  simple,  conical  imperforate,  non-tabulate  proto- 
type, or  protocorallum,  may  be  assumed  for  the  Madreporaria 
Perforata.  The  next  derived  form,  represented  by  the  early 
neanic  stages  of  P.  lenticulare,  has  the  structure  and  growth 
of  Aulopora,  and  consists  of  the  parent  cell  with  one  or  more 
buds.  At  this  stage,  which  may  be  called  the  Aulopora 
stage,  the  initial  corallite  has  the  same  number  of  mural  pores 
as  developed  buds,  for  each  bud  leads  into  the  parent  cell  by 
a  basal  opening  or  pore.  Aulopora  may  thus  be  considered 
as  representing  a  primitive  type  of  a  poriferous  coral,  in 
which  the  number  of  pores  in  each  corallite  corresponds  to 
the  number  of  buds  given  off  plus  one  connecting  it  with  the 
parent  cell.  Some  species  of  this  genus  are  free  throughout 
most  of  their  growth  (A.  subtenuis  Hall),  agreeing  closely 
with  the  erect  growth  of  Romingeria  and  Syringopora.  This 
fact  removes  one  of  the  important  arguments  against  the 
relations  of  Aulopora  with  these  genera.  The  corallites  of 
Aulopora  usually  send  off  buds  before  turning  out  of  the 

*  Amer.  Jour.  Sci.  (3),  III,  287,  March,  1872. 


428  STUDIES  IN  EVOLUTION 

common  axis  of  the  branch  or  colony,  after  which  no  gem- 
mation commonly  takes  place.  By  the  explanation  here 
advanced  this  lack  of  a  tendency  to  gemmation  in  the  distal 
portions  of  the  coral lites  in  this  genus  accounts  for  the  ab- 
sence of  mural  pores  when  such  portions  are  in  contiguity. 
The  periods  of  gemmation  in  Romingeria  are  periodic.  Sev- 
eral buds,  often  forming  a  verticil,  are  given  off  from  the 
parent  corallite.  Considerable  elongation  of  the  tubes  takes 
place  before  other  series  of  buds  are  produced.  The  budding 
is  prolific  at  these  points,  and  here  also  occur  the  mural  pores. 
The  latter  are  therefore  developed  when  the  period  of  gem- 
mation is  in  force.  If  pores  are  formed  elsewhere  when  the 
corallites  happen  to  come  into  juxtaposition,  it  may  possibly 
be  explained  as  the  result  of  a  stimulus  produced  by  the 
contiguity  of  the  animals.  Further  observations  are  neces- 
sary to  show  that  pores  exist  at  other  places  than  the  bases  of 
the  verticils  or  points  where  numerous  buds  are  given  off  and 
where  from  crowding  the  corallites  are  in  juxtaposition. 

It  therefore  seems  that,  primarily,  the  development  of 
mural  pores  is  identical  or  homologous  with  the  process  of 
gemmation.  Whether  this  cause  is  operative  in  such  forms 
as  Columnopora  or  Alevopora  yet  remains  for  investigation. 
The  porous  condition  of  the  walls  in  these  genera  may  be  an 
inherited  character  without  an  active  exciting  cause,  or  it 
may  be  teleologically  different. 


2.    SYMMETRICAL   CELL   DEVELOPMENT  IN 
THE   FAVOSITID^E* 

(PLATES  XXXII  AND  XXXIII) 

THE  majority  of  compound  corals  included  in  the  Favositidse 
are  composed  of  polygonal  prismatic  cells  or  corallites  in  juxta- 
position. When,  however,  these  cells  become  free,  their  form 
is  cylindrical.  The  polygonal  form  of  closely  arranged  cells 
is  therefore  explained  as  the  natural  result  of  crowding. 

The  species  Pleurodictyum  lenticular e  Hall,  sp.,  is  an 
example  of  simple  cell  growth  and  multiplication.  In  the 
development  of  this  species,,  as  shown  by  the  writer  in 
the  previous  paper,  the  initial  corallite  is  first  conical.  The 
growth  of  a  peripheral  series  of  buds  results  in  changing  the 
sub-circular  section  of  the  parent  corallite  into  a  polygon. 
The  buds  are  angular  on  the  sides  in  juxtaposition  to  the 
parent  cell  and  adjacent  buds,  but  on  the  free  portion  of 
their  periphery  they  are  cylindrical.  The  subsequent  growth 
of  peripheral  buds  brings  the  first  series  wholly  within  the 
corallum,  and  they  are  then  polygonal  in  section  like  the 
parent  corallite. 

In  compact  corals  with  long  cell  tubes,  as  Michelinia  and 
Favosites,  there  is  a  maximum  limit  to  the  size  of  the  coral- 
lites. Thus  the  form  of  the  cells  which  have  reached  this 
limit  of  diametral  extension  is  that  of  equal  hexagonal  prisms. 
This  is  of  course  due  to  the  well-known  fact  of  six  equal 
tangent  circles  about  a  central  circle  of  the  same  size.  Then 
from  crowding,  or  from  the  elimination  of  the  interstitial 
spaces,  they  assume  a  regular  hexagonal  form.  The  speci- 

*  Trans.  Conn.  Acad.  Sci.,  VIII,  215-220,  pis.  xiv,  xv,  1891. 


430  STUDIES  IN  EVOLUTION 

men  of  Cleistopora  geometrica,  illustrated  by  Edwards  and 
Haime,  *  represents  the  maximum  size  of  the  cells  and  their 
equal  development  in  this  species.  Although  the  tubes  are 
not  long,  the  calices  are  nearly  of  the  same  size,  and  regu- 
larly hexagonal. 

After  the  completion  of  a  circle  of  calices  about  the  parent 
cell  of  the  corallum  enlargement  takes  place,  (1)  by  buds 
from  the  periphery,  and  (2)  by  intermural  gemmation.  The 
first  is  not  attended  by  any  phenomena  differing  from  the 
production  of  the  primary  circlet  of  calices  about  the  initial 
cell.  The  second  takes  place  under  other  conditions,  and  is 
the  chief  method  of  increase  in  the  growth  of  large  corallums 
having  numerous  corallites. 

The  radial  arrangement  of  the  tubes  in  a  large  hem- 
ispherical or  cylindrical  mass  tends  to  make  the  axes  of  the 
corallites  diverge.  This  divergence  can  be  taken  up  only  by 
an  increase  in  the  diameters  of  the  tubes,  or  by  the  addition 
of  new  calices  between  the  others.  The  latter  mode  is  called 
intermural  gemmation.  In  Favosites  and  allied  genera  the 
maximum  size  of  the  corallites  is  soon  reached,  and  the  ex- 
pansion of  the  coral  is  mainly  derived  from  intermural  growth. 
The  study  of  this  method  of  increase  properly  begins  after 
one  or  more  rows  of  calices  have  been  developed  about  the 
parent  cell,  and  the  calices  have  reached  their  full  dimensions. 

The  following  description  of  a  symmetrical  system  of  in- 
termural cell  multiplication  was  observed  in  a  hemispherical 
specimen  of  Michelinia  convexa  D'Orbigny,  from  the  Cornifer- 
ous  Limestone  of  the  Falls  of  the  Ohio.  It  shows  very  clearly 
the  stages  of  development  of  the  interstitial  buds,  and  their 
modifications.  Other  corals  were  examined  to  the  same  end, 
and  were  found  to  agree  in  all  essential  particulars,  whenever 
their  growth  was  not  irregular  from  their  condition  of  fix- 
ation, or  from  the  excessive  development  or  death  of  a  number 
of  the  corallites.  An  exact  number  of  peripheral  buds  is  not 

*  Monographic  des  Polypiers  Fossiles  des  Terraines  Palaeozoiques,  252,  pi.  17, 
fig.  3,  1851. 


SYMMETRICAL  CELL  DEVELOPMENT  IN  FA  VOSITIDM  431 

necessary  to  illustrate  the  general  laws  of  intermural  growth. 
The  buds  produced  from  any  given  cell  cannot  always  agree 
with  the  symmetrical  method  here  described,  on  account  of 
the  crowding  of  similar  series  from  adjacent  or  neighbor- 
ing corallites.  After  eliminating  these  variations,  it  was 
found  that  the  process  of  intermural  gemmation  in  general 
is  quite  uniform,  and  closely  conforms  to  that  in  Michelinia 
convexa. 

Plate  XXXII,  figure  1,  represents  diagrammatically  the 
top  of  a  corallum  composed  of  a  central  parent  cell  and  six 
equal  peripheral  buds,  making  seven  nearly  equal  calices  in 
the  corallum.  The  upward  growth  of  these  corallites  and  the 
divergence  due  to  the  direction  of  their  axes  tend  to  separate 
them  from  the  parent  cell.  In  consequence  of  this  separation 
of  the  corallites,  they  would  naturally  assume  a  cylindrical 
form,  and  there  would  thus  appear  triangular  interspaces 
between  the  tangent  points  of  any  three  adjacent  calices. 
These  angles,  therefore,  afford  the  only  opportunities  for  the 
introduction  of  a  set  of  intermural  buds,  and  their  initial 
triangular  form  is  determined  by  the  conditions  of  growth. 
The  smallest  number  of  buds  which  can  be  symmetrically 
placed,  and  compensate  for  the  divergence  of  the  corallites,  is 
three,  one  from  each  alternate  angle  of  the  hexagon  (Plate 
XXXII,  figure  2). 

If  these  interstitial  cells  were  to  grow  without  the  introduc- 
tion of  others,  until  the  original  peripheral  series  was  com- 
pletely separated  from  the  parent  or  central  cell,  there  would 
result  a  corallum  containing  only  triangular  corallites.  There 
is,  however,  a  manifest  tendency  of  the  organism  to  the 
production  and  maintenance  of  a  cylindrical  form,  or  of  a 
prism  with  nearly  equal  radial  axes,  as  in  a  hexagonal  or 
polygonal  prism.  To  accomplish  this,  and  further  to  take  up 
the  divergence  of  the  corallites,  three  new  interstitial  buds 
are  introduced  at  the  remaining  three  unmodified  angles,  as 
shown  in  figure  3.  At  this  stage  there  are  six  symmetrically 
disposed  triangular  buds,  or  intermural  cells,  about  the 
central  corallite,  truncating  its  original  angles,  and  making 


432  STUDIES  IN  EVOLUTION 

it  a  twelve-sided  prism.  This  stage  is  the  third  toward  the 
formation  of  a  series  of  mature  interstitial  calices. 

During  the  third  stage  the  intermural  buds  increase  in 
size  until  they  completely  surround  the  parent  cell.  Then 
further  growth  truncates  their  adjacent  angles,  thus  adding 
two  more  sides  to  each  bud,  making  them  pentagonal  in 
section.  This  marks  the  fourth  stage  of  intermural  growth. 
At  the  same  time  the  central  corallite  loses  six  of  its  sides, 
and  returns  to  its  early  hexagonal  form.  The  axes  have 
revolved  30°,  and  the  original  sides  have  now  become  the 
angles  of  the  corallite  (Plate  XXXII,  figure  4). 

At  this  period  of  growth  it  is  necessary  to  consider  a 
series  of  buds  on  the  periphery  of  the  corallum,  marked  ll\ 
2",  etc.,  in  Plate  XXXII,  figures  3  and  4.  They  are  first 
triangular  in  form  like  the  others,  and  of  two  sizes,  owing  to 
their  different  ages.  The  growth  of  this  series  continues 
until  they  touch  and  truncate  the  angles  of  the  first  series 
(!',  #',  etc.),  producing  the  fifth  condition  or  stage.  The 
first  series  of  buds  has  now  three  hexagonal  and  three 
pentagonal  corallites  (Plate  XXXII,  figure  5). 

In  the  last  or  sixth  stage  (figure  6),  the  further  growth  of 
all  the  intermural  cells  results  in  a  corallum  of  nineteen  nearly 
equal  hexagonal  corallites.  The  original  parent  cell  (A)  is 
at  the  centre,  the  first  six  intermural  cells  (!',  21,  etc.)  com- 
pletely surround  it,  and  the  six  new  peripheral  corallites 
(2",  0",  etc.)  are  interposed  between  the  members  of  the 
original  circlet  (7,  #,  etc.).  The  effect  of  this  intermural 
growth,  then,  is  to  dissociate  all  the  first  series  of  corallites 
from  the  parent  cell  and  from  each  other.  (See  Plate 
XXXIII.) 

The  changes  taking  place  in  the  number  and  form  of  the 
cells  may  be  tabulated  as  on  page  433. 

Buds  are-  developed  in  Favosites  and  Michelinia  whenever 
there  is  a  space  or  opportunity  for  their  growth,  unless  the 
corallum  is  affected  by  some  abnormal  condition.  If  this  ten- 
dency to  form  a  solid  mass  of  corallites  were  not  so  strong, 
and  if  the  process  of  budding  took  place  only  at  compara- 


SYMMETRICAL  CELL  DEVELOPMENT  IN  FA  VOSITID^E  433 


tively  remote  intervals,  the  corallum  would  have  the  form  of 
Romingeria.  In  Michelinia  convexa  it  is  evident  that,  if  the 
divergence  of  the  corallites  was  considerable  and  not  wholly 
filled  by  intermural  growths,  there  would  result  a  verticil  of 
corallites  about  the  parent  cell  which  would  soon  become 
free.  The  peripheral  corallites,  also,  would  become  separated. 
Then  after  further  growth  the  parent  cell  would  give  off 
another  verticil  of  buds,  the  other  corallites,  likewise,  develop 
similar  verticils,  and  the  whole  form  and  mode  of  growth 
would  be  like  that  of  Romingeria.  From  this  point  of  view, 
Romingeria  may  represent  an  early  form  of  symmetrical  cell 
development  in  the  poriferous  corals.  The  acceleration  of 
the  periods  of  gemmation  and  consequent  approximation  of 
the  corallites  carrying  their  verticils  of  buds  would  produce 
all  the  conditions  of  cell  growth  and  intermural  gemmation 
exhibited  by  Favosites  or  Michelinia. 


Stages. 

Form  of  primary  cell. 

Whole  No. 
of  cells. 

Number  of 
intermural 
buds. 

Number  of 
sides  of 
buds. 

Nepionic. 

cone. 

1 

0 

0 

First  completed  neanic 
or  first  condition  requi- 
site to  intermural  gem- 

\    6-sided  prism. 

7 

0 

0 

mation. 

j 

2d  stage. 

9-sided  prism. 

11 

3 

3 

3d  stage. 

12-sided  prism. 

16 

9 

3 

4th  stage. 

6-sided  prism. 

19 

19    *   6 
12  )   6 

.  .     3 
.  .      5 

I3 

3 

5th  stage. 

6-sided  prism. 

19 

12  \   3 

...  .4 
.  .  ..5 

I  3 

6 

6th  stage. 

6-sided  prism. 

19 

12 

6 

Summary. 

The  growth  of  intermural  buds  compensates  for  the  natural 
divergence  of  the  corallites.  New  cells  are  introduced  when- 
ever the  old  corallites  have  reached  their  maximum  size,  and 
when  their  divergence  approaches  a  separation  of  the  cell 
tubes. 

28 


434  STUDIES  IN  EVOLUTION 

The  form  of  the  buds  is  first  that  of  a  triangular  pyramid 
or  prism,  and  is  due  to  the  mechanical  conditions  of  growth. 
During  subsequent  increase  they  touch  and  truncate  each 
other,  changing  from  triangular  to  five-  and  six-sided  prisms. 
Complete  symmetrical  normal  development  produces  a  coral- 
lum  with  equal  hexagonal  calices.  The  process  of  intermural 
gemmation  changes  the  sides  of  the  parent  cells  to  angles, 
and  the  older  corallites,  originally  in  juxtaposition,  become 
separated  from  each  other  by  new  series  of  interstitial 
calices. 


3.  DEVELOPMENT  OF  THE  SHELL  IN  THE 
GENUS  TORNOCERAS  HYATT* 

(PLATE  XXXIV) 

THE  leading  embryonal  characters  of  the  genus  Tornoceras 
have  been  drawn  mainly  from  results  obtained  in  the  study  of 
Tornoceras  retrorsum  von  Buch,  and  allied  species  from  the 
Devonian  of  Germany,  -f-  Probably  the  best  study  of  any  one 
of  the  species  is  that  given  by  W.  Branco  of  T.  retrorsum,  var. 
typum  Sandberger.  f  The  adult  features  have  been  deter- 
mined from  the  type  T.  ( Groniatites)  uniangulare  Conrad,  and 
other  closely  related  forms.  Hitherto,  knowledge  of  this 
species  has  not  been  sufficient  to  give  a  reasonably  full 
diagnosis  of  the  genus  in  its  developmental  relations,  and 
the  results  of  the  following  study  aim  to  supply  the  deficiency. 
The  importance  of  this  is  evident,  as  the  characters  of  the 
type  are  of  prime  consequence,  and  because  T.  retrorsum 
offers  some  differences  in  its  development,  and  apparently 
belongs  to  one  of  the  more  advanced  phases  in  the  evolution 
of  the  generic  stock.  Instead  of  presenting  a  gradual  growth 
from  its  simple  nautilif orm  protoconch  through  several  slightly 
diverging  stages,  it  exhibits,  to  a  degree,  the  principle  of 
accelerated  development,  as  will  be  shown  hereafter ;  while 
T.  uniangulare  has  a  more  uniform  and  complete  growth,  and 
is  probably  one  of  the  initial  and  most  primitive  species  of 
the  genus. 

*  Amcr.  Jour.  Sci.  (3),  XL,  71-75,  pi.  i,  1890. 

t  Proc.  Boston  Soc.  Nat.  Hist.,  XXII,  Hyatt.  Genera  of  Fossil  Cephalopods, 
320,  1883. 

J  Palwontographica,  XXVII.  Beitrage  zur  Entwickelungsgeschichte  der 
fossilen  Cephalopoden,  1880. 


436  STUDIES  IN  EVOLUTION 

Besides  the  sutural  and  tubular  development,  the  material 
studied  illustrates  the  inception  and  growth  of  the  surface 
ornaments,  and  as  these  features  are  rarely  found,  the  princi- 
ples involved  are  of  more  than  generic  application. 

The  specific  limits  of  T.  uniangulare  have  not  been  clearly 
defined,  and  many  of  the  forms  referred  to  Parodiceras  (  Go- 
niatites)  discoideum  Hall,  are  evidently  of  the  former  species. 
A  comparison  of  the  type  specimens  of  both  with  others 
which  have  been  grouped  with  them,  as  figured  in  the 
Thirteenth  Report,  New  York  State  Cabinet,  and  in  Vol.  V, 
Part  II,  of  the  Palaeontology  of  New  York,  shows  that  the 
first  species  is  really  the  common  one,  and  as  far  as  known, 
the  second  is  represented  only  by  the  original  types.* 

The  adult  differences  are  mainly  noticeable  in  the  depth  of 
the  air-chambers  and  in  the  sutural  curves.  They  can  readily 
be  determined  by  strictly  limiting  the  characters  to  those  first 
ascribed  to  each  species.  Parodiceras  discoideum  is  also 
apparently  without  the  narrow  cone  at  the  bottom  of  the 
annular  lobe,  and  the  ventral  saddle  is  much  less  depressed. 

The  material  for  this  paper  is  a  portion  of  a  collection  pre- 
sented to  the  museum  of  Yale  University  by  Thomas  G.  Lee, 
M.D.  The  particular  lot  containing  the  Tornoceras  consisted 
of  several  hundred  nodular  concretions  of  pyrite  of  a  radiated 
structure,  obtained  from  the  Devonian  (Hamilton)  shales  of 
Wende  Station,  Erie  county,  New  York.  Most  of  them 
preserved  an  organic  nucleus,  and  about  twenty-five  species 
have  been  identified  as  belonging  to  the  Trilobita,  Cephalopoda, 
Pteropoda,  Pelecypoda,  Brachiopoda,  and  Crinoidea. 

The  test  of  the  trilobites  and  the  shells  of  the  brachiopods 
are  but  little  altered,  while  those  of  the  cephalopods  and 
pelecypods  are  usually  replaced  by  sphalerite,  a  difference 

*  The  following  list  is  proposed  as  corrected  references  to  T.  uniangulare : 

13th  Ann.  Rept.  N.  Y.  State  Cab.,  98,  figs.  6  (bis)  (type  specimen)  and  6? 

Pal.  N.  r.,  V,  pt.  ii,  pi.  71,  figs.  11-14  (fig.  14  =  type  specimen) ;  pi.  72,  figs. 
6,  7 ;  pi.  74,  figs.  2,  4  ;  VII,  pi.  127,  figs.  10,  11,  12. 

Of  these,  pi.  71,  figs.  11,  12,  13 ;  pi.  74,  fig.  4  and  pi.  127,  figs.  11,  12,  were 
referred  to  P.  (Gomatites)  discoideum  Hall. 


DEVELOPMENT  OF  THE  SHELL  IN  TORNOCERAS    437 

evidently  connected  with  the   more   soluble   nature   of   the 
pearly  shells  of  the  nuculoids  and  cephalopods. 

By  carefully  breaking  away  the  outer  enveloping  volutions 
of  a  number  of  specimens  of  Tornoceras,  the  early  parts  of 
the  shell  were  uncovered,  and  found  to  be  well  preserved,  and 
therefore  suitable  for  study.  The  drawings  on  Plate  XXXIV 
were  made  from  the  microscope,  with  a  camera  lucida. 

The  protoconch  (figures  1,  2,  Plate  XXXIV)  has  an  axial 
diameter  of  about  1.1  mm.,  and  among  several  specimens 
measured  varies  but  little  from  this  dimension.  The  verti- 
cal diameter  is  a  little  shorter,  so  that  the  general  form  is 
that  of  a  prolate  ellipsoid.  The  latera  are  prominent,  and 
exposed  as  central  bosses  in  the  umbilicus  of  a  young  shell. 

At  what  precise  growth-stage  the  umbilicus  becomes  closed 
cannot  be  ascertained  from  the  material  studied,  but  it  is  evi- 
dently open  during  the  formation  of  several  whorls.  During 
the  concrescence  of  the  first  few  air-chambers,  while  the 
diameter  of  the  tube  is  diminishing,  the  tendency  of  the 
umbilicus  is  to  enlarge  rapidly.  Subsequent  increase  in  the 
tube  and  the  greater  involution  of  the  whorls  contract  it,  so 
that  in  adult  specimens  it  is  closed,  while  in  large  and  often 
senile  individuals  a  secondary  deposit  is  made  about  the 
umbilicus,  entirely  obliterating  it  and  covering  the  growth 
lines  of  the  shell.*  Evidently  this  formation  is  similar  to 
that  deposited  by  the  dorsal  lobe  of  the  mantle  in  Nautilus 
pompilius. 

The  axial  diameter  of  the  embryo  shell  is  somewhat  greater 
than  that  of  several  of  the  succeeding  air-chambers.  Thus, 
in  its  growth,  the  tube  first  contracts,  and  does  not  assume 
the  regular  rate  of  increase  until  after  the  formation  of  at 
least  the  second  septum.  A  cross  section  at  the  first  septum 
is  transversely  sub-elliptical,  slightly  arcuate,  with  a  longer 
diameter  two  and  one-third  times  greater  than  the  shorter. 
When  a  transverse  diameter  of  1.2  mm.  is  reached  by  the 

*  This  feature  is  well  represented  in  fig.  11,  pi.  127,  Pal.  N.  Y.,  VII, 
supplement. 


438  STUDIES  IN  EVOLUTION 

larval  shell,  the  outline  of  a  section  is  lunate,  but  the  pro- 
portions of  length  and  height  are  not  materially  changed. 
A  section  of  the  adjacent  whorl  is  still  more  arcuate,  as 
shown  in  figure  6,  and  in  an  adolescent  specimen  11.5  mm.  in 
diameter  it  is  seen  that  the  diametral  relations  have  become 
interchanged,  and  that  the  outer  whorl  is  elliptical  in  a  verti- 
cal direction,  and  excavated  by  the  inner  whorl  to  nearly  half 
its  longest  diameter,  making  the  shell  in  all  neanic  and 
ephebic  stages  decidedly  compressed  in  outline  (figure  13). 

The  first  septum  (figures  1,  2,  4)  is  moderately  concave, 
and  extends  nearly  to  the  axis.  The  suture  is  simple,  being 
nearly  in  a  single  plane,  without  apparent  lobes  or  saddles. 
Occasionally  the  internal  mould  shows  a  siphonal  lobe  due  to 
the  breaking  away  of  the  extremely  thin  filling  between  the 
siphon  and  ventrum,  but  perfect  specimens  determine  this  to 
be  an  accidental  condition. 

In  the  section  represented  in  figure  12,  the  first  two  septa 
are  much  thicker  than  those  immediately  succeeding,  a  fea- 
ture also  noticeable  on  the  exterior  of  the  internal  mould. 
Likewise  the  first  and  second  air-chambers  are  deeper  than 
the  three  or  four  following.  With  these  exceptions  the  septa 
and  air-chambers  are  generally  uniform  in  their  progression 
until  the  adult  stage. 

It  has  been  already  noted  that  the  first  septum  is  extremely 
simple,  without  apparent  lobes  or  saddles.  In  the  second  sep- 
tum there  is  a  well-developed  sinus  over  the  siphuncle,  form- 
ing a  rounded  ventral  lobe,  and  a  broad  lateral  saddle,  with  a 
corresponding,  though  less  prominent,  dorsal  saddle.  The 
third  and  following  septa  present  more  and  more  sharply 
angular  ventral  lobes,  until  finally  it  is  further  extended  by  a 
siphonal  fissure  in  post-nepionic  stages.  The  lateral  saddle 
is  not  so  strongly  curved  from  the  fourth  to  the  seventh 
suture,  which  is  quite  flat,  but  in  the  eighth  a  slight  retral 
bend  is  observable.  This  evidently  marks  the  inception  of 
the  lateral  lobe.  The  septum  is  now  divided  into  the  leading 
members  characteristic  of  the  group ;  namely,  a  ventral  lobe 
and  saddle,  a  lateral  lobe  and  saddle,  a  dorsal  saddle  and  an 


DEVELOPMENT  OF  THE  SHELL  IN  TORNOCERAS    439 

annular  lobe,  although  the  two  latter  are  less  strongly  marked 
than  the  others.  Further  growth  merely  serves  to  emphasize 
these  features,  until  the  neanic  stadium,  when  the  ventral 
lobe  is  extended  by  the  siphonal  fissure,  and  a  minute  cone 
appears  at  the  bottom  of  the  annular  lobe. 

Several  specimens  of  the  protoconch  give  evidence  of  the 
presence  of  the  siphonal  caecum,  and  show  that  it  was 
probably  closely  appressed  to  the  ventral  wall.  Figure  3 
illustrates  the  ovoid  marking  on  the  interior  of  the  shell, 
enclosing  two  diverging  lines  which  apparently  represent  the 
appressed  portion  of  the  true  caecum,  while  the  outer  curved 
lines  limit  the  shelly  deposit  of  attachment.  The  relative 
diameter  of  the  siphon  at  the  first  and  for  a  number  of  suc- 
ceeding chambers  is  much  greater  than  in  the  mature  shell 
(figures  1,  6,  and  13).  From  the  beginning,  it  is  situated 
close  to  the  abdominal  wall,  and  is  nearly  invariable  in  its 
character. 

The  embryonic  shell  is  very  thin,  and  almost  smooth  in  its 
earlier  portions  ;  then  fine  revolving  lines  of  granules  appear, 
which  become  progressively  more  pronounced  and  arranged 
in  transverse  rows,  between  which  the  earliest  of  the  concen- 
tric strise  are  developed.  With  the  increase  in  the  strength 
of  the  strise,  the  granules  disappear,  and  are  obsolescent 
before  the  protoconch  is  completed  (figure  7).  The  strise  are 
sharp,  elevated,  and  straight,  forming  a  conspicuous  feature 
of  the  ornamentation,  until  in  the  third  or  fourth  whorl, 
when  they  become  subdued,  and  finally  are  replaced  by  the 
fine  inconspicuous  and  often  fasciculate  lines  of  growth  which 
are  present  in  the  adult  shell.  No  indication  of  a  funnel  is 
shown  up  to  the  completion  of  the  first  whorl  (figure  10),  as 
the  strise  continue  straight  across  the  ventrum,  but  in  the 
second  (figure  11)  the  pronounced  sinus  in  the  strise  shows 
that  the  funnel  had  developed  or,  at  least,  had  become  of 
functional  importance. 

A  comparison  of  the  figures  of  T.  retrorsum  von  Buch,  var. 
typum  Sandberger,  as  illustrated  by  Branco  (loc.  cit.,  pi.  v, 
fig.  vii),  with  the  present  species  shows  that  the  former 


440  STUDIES  IN  EVOLUTION 

presents  a  more  arcuate  first  septum,  and  that  the  second  is 
comparable  with  the  third  or  fourth  of  T.  uniangulare,  clearly 
indicating  that  the  development  has  been  accelerated  by  the 
skipping  of  at  least  two  phases  of  growth  in  the  septa.  In 
other  characters  the  two  forms  merely  indicate  differences 
which  are  probably  only  of  specific  importance,  such  as  the 
more  angular  form  of  the  lobes  and  saddles  in  T.  retrorsum, 
var.  typum,  and  the  absence  of  the  minute  cone  at  the  summit 
of  the  annular  lobe. 


PLATES  AND  EXPLANATIONS 


PLATE  I 


PLATE   I 


SPINES   OF  RADIOLARIA   (PAGE  16) 

Spiniform  processes  of  recent  Radiolaria  taken  from  the  shells  of  the 
following  species  :  — 


FIGURE  1.  —  Heliosphcera  coronata.    FIGURE  29.  — 

"       2.  —  Astrosphcera  stellata.  "  30.  — 
"       3.  —  Astrophacus  Solaris. 

"       4.  —  Stylosphcera  calliope.  "  31.  — 

"       5.  —  Heliodiscus  glyphodon.  "  32.  — 
"       6.  —  Pripodictya  triacantha. 

"       7.  —  Pleuraspis  horrida.  "  33.  — 

"        8.  —  Hexacontium  sceptrum.  "  34.  — 

"       9. — Acanthosphcera  clavata.  "  35. — 

"      10. — Acanthosphcera  clavata.  "  36. — 

"      11.  —  Cromyodrymus  quadri-  "  37.  — 

cuspis. 

"     12.  —  Hexacontium     clavige-  "  38.  — 

rum.  "  39.  — 

«      13.  _.  Orosphcera  horrida.  "  40.  — 

"      14.  —  Staurocycliaphacostau-  "  41.  — 

rus. 

"      15.  —  Tripospyris  capitata.  "  42.  — 

"      16. — A rchipera  cortiniscus.  "  43. — 
"      17.  —  Tripospyris  conifera. 

"      18.  —  Orosphcera  serpentina.  "  44.  — 

"      19.  —  Staurolonche  pelusa.  "  45. — 
"     20.  —  Astrosphcera  stellata. 

"     21.  —  Staurodictya  elegans.  "  46.  — 

"     22.  —  Hexastylus  contortus.  "  47.  — 

"     23.  —  Stephanospyris      excel-  "  48.  — 

lens. 

"     24.  —  Podocyrtis  magnifica.  "  49.  — 

"      25.  —  Hexancistra  mirabilis.  "  50.  — 

"     26.  —  Dictyophimw  Cienkows-  "  51. 

M.  "  52. 
"     27.  —  Elafomma  Juniperinum. 
"     28.  —  Castanura  Tizardi. 


Pleuraspis  horrida. 
Staurocarynum  arbores- 
cens. 

—  Rhizosphwra  serrata. 

—  Phcenocalpis   petalospy- 

ris. 

—  A  ulospathis  bifurca. 

—  Auloyraphis  bovicornis. 

—  Aulographis  ancorata. 

—  Aulographis  bovicornis. 

—  Sphcerozoum     verticilla- 

tum. 

—  Cladococcus  pinetum. 

—  Hexancistra  triserrata. 

—  Cladococcus  stalactites. 

—  Hexancislra    quadricus- 

pis. 

—  Heliodrymus  ramosus. 

—  Heliodrymus    dendrocy- 

clus. 

—  A  ulograph  is  pandora. 

—  Cladoscenium      ancora- 

tum. 

—  Cladococcus  scoparius. 

—  Auloscena  penicillus. 

—  Circostephanus    corona- 

rius. 

—  Lychnospho3ra  regina. 

—  A  uloscena  spectabilis. 
—  Cozlospathis  ancorata. 

—  Octodendron    spathilla- 

tum. 


1234        567 


Plate  I 

10     11      12 


13        14 


15       16       17       18      19      20      21      22 


23     24       25       26      27 


29      30      31     32       33 


SPINES  OP  RADIOLARIA 


f*' 

/v 

/  ur    ii-it.. 

INIVERSITYJ 

\:-,; 


PLATE  II 


PLATE   II 
DEVELOPMENT   AND   CLASSIFICATION  OF   TRILOBITES 

Figures  are  approximately  of  the  same  size  to  facilitate  comparison. 
The  dorsal  areas  of  the  free-cheeks  are  shaded. 

Ontogeny  of  Sao  Ursula  Barrande  (Pages  128,  129) 

FIGURE  1.  —  Protaspis  stage. 

FIGURE  2.  —  Cephalon ;  nepionic  stage  of  individual  having  two  free 
thoracic  segments. 

FIGURE  3.  —  Cephalon;  later  nepionic  stage  of  individual  having  eight 
free  thoracic  segments. 

FIGURE  4.  —  Cephalon  of  adult.     (Figures  1-4,  after  Barrande.) 

Ontogeny  of  Dalmanites  socialis  Barrande  (Pages  128,  129) 

FIGURE  5.  —  Protaspis  stage. 

FIGURE  6.  —  Cephalon ;  nepionic  stage  of  individual  having  three 
free  thoracic  segments. 

FIGURE  7. —  Cephalon;  nepionic  stage  of  individual  having  seven 
free  thoracic  segments. 

FIGURE  8.  —  Cephalon  of  adult.     (Figures  5-8,  after  Barrande.) 

HYPOPARIA  (Pages  134-138) 

FIGURE     9.  —  Cephalon  of  A qnostus        )   .          ,., 
FIGURE  10.  —  Cephalon  of  Microdiscus  \  Agno 
FIGURE  11.  —  Cephalon  of  Harpes  }-  Harpedidse. 

FIGURE  12.  —  Cephalon  of  Trinucleus    1  T  •   ,    n  . , 
FIGURE  13.  -  Cephalon  of  Ampyx          \  Trmucleidae. 

OPISTHOPARIA  (Pages  138-152) 

FIGURE  it.  -Cephalon  of  ^onocoryphe   |  Conocoryphidse. 
FIGURE  16.  —  Cephalon  of  Ptychoparia   >  ni     . , 
FIGURE  17.  —  Cephalon  of  Olenus  Ule     18e* 


FIGURE  18.  —  Cephalon  of  Asaphus 
FIGURE  19.  —  Cephalon  of  Illcenus 
FIGURE  20.  —  Cephalon  of  Proetus 
FIGURE  21.  —  Cephalon  of  Bronteus 
FIGURE  22.  —  Cephalon  of  Lichas 


Asaphidse. 

Proetidse. 
Bronte  idae. 
Lichadidse. 


FIGURE  23.  —  Cephalon  of  Acidaspis        j- Acidaspidse. 
PROPARIA  (Pages  152-157) 

FIGURE  24.  -Cephalon  of  Placoparia  >  Encrinuridffi. 

FIGURE  2o.  —  Cephalon  of  Encnnurus  > 

FIGURE  26.  —  Cephalon  of  Calymmene  )  Calvmmenidse 

FIGURE  27.  —  Cephalon  of  Dipleura      \        J 

FIGURE  28.  —  Cephalon  of  Cheirurus  (Eccoplocheile)      }-Cheiruridse. 

FIGURE  29.  —  Cephalon  of  Dalmanites 

FIGURE  30.  —  Cephalon  of  Dalmanites 


FIGURE  31.  — Cephalon  of  Chasmop* 
FIGURE  32.  —  Cephalon  of  A  caste 
FIGURE  33.  — Cephalon  of  Phacops 


Phacopidse. 


2  Sao 


3    Sao 


1    Sao 


6    Dalmanites 


6     Dalmanites 


7    Dalmanites 


Plate 


4    Sao 


8     Dalmanites 


HYPOPARIA 


12   Trinucleus  I          '13    Ampyx 


OPISTHOPARrA 


14    Atops  15  Conocoryphe       f  16   Ptychoparia  ^f    f     17     Olenus      H         18    Asaphus 


19    Iltenus  V    20    Proetus    "  21  Bronteus  22    Lichas  '     23    Acidaspis 


PROPARIA 


24    Placoparia  25  Encrinurus  26  Calymmene  27   Dipleura  28   Cheirurus 


Dalmanites  \ I     |/  31   ChasmonsM  32    Acaste  33   Phacops 

CLASSIFICATION  OF  TRILOBITES 


PLATE   III 


PLATE   III 
LARVAL   STAGES   OF   TRILOBITES   (PAGES  171-173) 

FIGURE  1.  —  Solenopleura  Robbi  Hartt.  (After  Matthew.)  Anapro- 
taspis  stage;  showing  obscurely  annulated  axis.  X  30.  St.  John  Group, 
Cambrian,  New  Brunswick. 

FIGURE  2.  —  Liostracus  onangondianus  Hartt.  (After  Matthew.) 
Anaprotaspis  stage;  the  neck  lobe  is  the  only  one  distinctly  marked. 
X  23.  Cambrian,  New  Brunswick. 

FiGURE3.  —  Ptychoparia  Linnarssoni  Walcott.  (After  Matthew.) 
Anaprotaspis  stage;  axis  slender,  slightly  annulated;  pygidium  defined 
by  transverse  furrow.  X  30.  Cambrian,  New  Brunswick. 

FIGURE  4.  —  Ptychoparia  Linnarssoni,  Walcott.  (After  Matthew.) 
Protaspis  ;  representing  a  later  moult  than  the  preceding,  and  showing 
stronger  annulations  on  the  axis,  with  an  additional  one  on  the  pygidium. 
X  25.  Cambrian,  New  Brunswick. 

FIGURE  5.  —  Ptychoparia  Kingi  Meek.  Anaprotaspis  or  early  stage  ; 
showing  obscurely  defined  characters,  partly  due  to  the  fact  that  the 
specimen  is  a  cast.  X  45.  Cambrian,  Nevada. 

FIGURE  6.  —  Ptychoparia  Kingi  Meek.  A  later  stage  (metaprotaspis)  ; 
showing  the  strongly  annulated  axis,  the  eye-line,  the  free-cheeks  includ- 
ing the  genal  angles,  and  two  segments  on  the  pygidium.  X  45.  Cam- 
brian, Nevada. 

FIGURE  7.  —  Ptychoparia  Kingi  Meek.  (After  Walcott.)  An  adult 
specimen.  This  and  the  other  figures  of  adult  individuals  are  represented 
in  outline,  with  the  free-cheeks  shaded,  to  bring  out  more  strongly  the 
changes  in  the  structure  of  the  cephalon.  X  J.  Cambrian,  Utah. 

FIGURES.  —  Sao  hirsuta  Barrande.  (After  Barrande.)  Anaprotaspis 
stage;  showing  obscurely  the  limits  of  the  pygidium,  the  eye-line,  and 
the  nearly  cylindrical  glabellar  axis  expanding  on  the  frontal  margin. 
This  and  the  two  following  specimens  are  preserved  as  casts.  X  30. 
Cambrian,  Bohemia. 

FIGURE  9.  —  Sao  hirsuta  Barrande.  (After  Barrande.)  A  later  moult, 
probably  near  the  end  of  the  metaprotaspis  stage ;  showing  the  annulated 
axis  expanded  in  front;  free-cheeks  narrow  and  marginal  ;  pygidium 
of  four  segments,  with  pleura  distinctly  marked  and  grooved.  X  30. 
Cambrian,  Bohemia. 

FIGURE  10. —  Sao  hirsuta  Barrande.  (After  Barrande.)  A  more  ad- 
vanced stage  at  or  after  the  close  of  the  paraprotaspis,  in  which  the 
pygidium  is  complete,  but  before  the  first  free  thoracic  segment  is  de- 
veloped. X  30.  Cambrian,  Bohemia. 

FIGURE  11.  —  Sao  hirsuta  Barrande.  An  adult  individual  combining 
the  characters  as  shown  in  several  of  Barrande's  figures  of  this  species. 
X  A.  Cambrian,  Bohemia. 

FIGURE  12.  —  Triarthrus  Becki  Green.  Anaprotaspis;  showing  the 
annulated  axis,  terminating  before  reaching  the  anterior  margin ;  the 
eye-lines  extending  from  the  first  segment  to  the  marginal  eye-lobes ; 
pygidium  defined  by  a  slight  groove,  and  including  two  segments  of  the 
axis.  X  45.  Ordovician,  Utica  Slate,  near  Rome,  New  York. 

FIGURE  13.  —  Triarthrus  Becki  Green.  Protaspis  at  a  later  moult ; 
showing  slight  increase  in  size  and  the  addition  of  a  segment  to  the 
pygidium.  X  45.  Ordovician,  Utica  Slate,  near  Rome,  New  York. 

FIGURE  14.  —  Triarthrus  Becki  Green.  An  adult  individual  of  this 
species.  X  £•  Ordovician,  Utica  Slate,  near  Rome,  New  York. 


11 


Plate 
4 


LARVAL  STAGES  OP  TRILOBITES 


PLATE  IV 


PLATE    IV 
LARVAL  STAGES  OF   TRILOBITES   (PAGES  173-170) 

FIGURE  1. — Acidaspis  tuberculata  Conrad.  Anaprotaspis  ;  showing 
denticulate  margin  and  spines  on  cephalon ;  axis  strongly  annulated ; 
eyes  sub-marginal.  X  20.  Lower  Helderberg,  Albany  county,  New  York. 

FIGURE  2.  — The  same;  profile,  slightly  oblique.     X  20. 

FIGURE  3. — Acidaspis  tuberculata  Conrad.  An  adult  individual,  re- 
stored from  fragments  and  an  entire  enrolled  specimen.  Natural  size. 
Lower  Helderberg,  Albany  county,  New  York. 

FIGURE  4. — Arges  consangmneus  Clarke.  Dorsal  view  of  a  larva  at 
or  after  the  close  of  the  paraprotaspis  stage ;  showing  the  form  and  orna- 
mentation. X  20.  Lower  Helderberg,  Albany  county,  New  York. 

FIGURE  5.  —  Proetus  parviusculus  Hall.  Anaprotaspis;  showing 
strongly  annulated  axis,  with  groove  at  each  side;  large  prominent 
anterior  eyes  ;  pygidial  pleura  indicated  by  faint  grooves,  x  45.  Ordo- 
vician,  Utica  Slate,  near  Rome,  New  York. 

FIGURE  6.  — Proetus  parviusculus  Hall.  A  later  moult,  near  the  close 
of  the  paraprotaspis  stage;  showing  the  larger  pygidium  which,  how- 
ever, is  still  incomplete,  and  the  slight  backward  movement  of  the  eyes. 
The  right  side  of  the  specimen  is  restored,  x  45.  Ordovician,  Utica 
Slate,  near  Rome,  New  York. 

FIGURE  7.  —  Proetus  parviusculus  Hall.  An  adult  individual.  X'-. 
Ordovician,  Utica  Slate,  near  Rome,  New  York. 

FIGURE  8.  —  Dalmanites  socialis  Barrande.  (After  Barrande.)  Ana- 
protaspis stage;  showing  the  large  strongly  annulated  axis;  the  prom- 
inent anterior  marginal  eyes ;  mucronate  genal  angles ;  pygidium  of 
three  segments.  X  30.  Ordovician,  Bohemia. 

FIGURE  9. — Dalmanites  socialis  Barrande.  (After  Barrande.)  Meta- 
protaspis  stage ;  showing  the  stronger  definition  of  the  pleura  of  the 
pygidium.  X  30.  Ordovician,  Bohemia. 

FIGURE  10.  —  Dalmanites  socialis  Barrande.  (After  Barrande.)  The 
specimen  probably  represents  the  close  of  the  paraprotaspis  stage,  and 
shows  four  segments  in  the  pygidium  and  the  first  evidence  of  the 
backward  movement  of  the  eyes,  which  now  indent  the  margin.  X  30. 
Ordovician,  Bohemia. 

FIGURE  11.  —  Dalmanites  socialis  Barrande.  (After  Barrande.)  Out- 
line of  an  adult  individual.  X  ^.  Ordovician,  Bohemia. 


Plate  IV 


LARVAL  STAGES  OF  TRILOBITES 


PLATE  V 


PLATE   V 

CRUSTACEAN   LARV^   (PAGES  191,  192) 

The  Roman  numerals  indicate  the  appendages  in  their  consecutive 
order. 

/,  1st  pair  of  appendages,  or  antennules. 
//,  2d  pair  of  appendages,  or  antennae. 

///,  3d  pair  of  appendages,  or  mandibles. 

IV,  F,  etc.,  maxillae,  maxillipeds,  swimming  feet,  etc. 

ocl,  unpaired  eye  ;  oc,  paired  eyes ;  Ib,  labrurn. 

FIGURE  1.  —  Triarthrus  Becki.  A  restoration  of  the  ventral  side  of 
the  protaspis  stage  in  accordance  with  the  best  evidence  at  present  at- 
tainable, as  explained  in  the  text.  The  F/th  and  the  F//th  pairs  of  ap- 
pendages belong  to  the  abdomen,  which  is  marked  oft'  by  a  transverse 
line;  mt,  metastoma;  g,  free-cheeks. 

FIGURE  2.  — Apus  cancriformis.  (After  Claus,  from  Faxon.)  Phyllop- 
oda.  Nauplius  larva,  just  hatched  ;  ventral  side.  Behind  the  mandibles 
(III}  are  indications  of  five  thoracic  somites  (y). 

FIGURE  3.  — Apus  cancriformis.  (After  Claus,  from  Faxon.)  Phyllop- 
oda.  Second  larval  stage  (metanauplius) ;  ventral  side.  The  second 
maxilla  (  V}  is  wanting ;  /,  frontal  sense  organs. 

FIGURE  4.  —  Branchipus  stagnalis.  (After  Claus,  from  Packard.) 
Phyllopoda.  Nauplius  stage. 

FIGURES.  —  Artemia  yracilis.  (After  Packard.)  Phyllopoda.  Nau- 
plius stage  ;  showing  obscure  segmentation. 

FIGURE  6.  —  Limnaida  Hermanni.  (After  Lereboullet,  from  Packard.) 
Phyllopoda.  Nauplius ;  dorsal  side ;  first  pair  of  appendages  obsoles- 
cent ;  labrum  (Ib)  greatly  developed. 

FIGURE  7.  —  Lepidurus  productus.  (After  Brauer,  from  Bernard.) 
Phyllopoda.  Nauplius  with  obscure  segmentation  of  the  trunk  (y). 

FIGURE  8.  —  Leptodara  hyalina.  (After  Sars,  from  Half  our  and  Bronn.) 
Phyllopoda,  Cladocera.  Nauplius  larva  from  winter  egg  ;  y,  rudimentary 
feet. 

FIGURE  9.  —  Daplmia  longhpina.  (After  Dohrn,  from  Claus.)  Phyllop- 
oda, Cladocera.  Nauplius  stage  of  embryo,  with  rudimentary  append- 
ages. 

FIGURE  10.  —  Moina  rectirostris .  (After  Grobben,  from  Faxon).  Phyl- 
lopoda, Cladocera.  Embryo  from  the  summer  egg  in  the  nauplius  stage, 
developed  in  the  brood-cavity  of  the  parent ;  appendages  rudimentary. 

FIGURE  11. —  Cyclops  tenuicornis.  (After  Claus,  from  Balfour.)  Co- 
pepoda,  Natantia.  Nauplius,  first  stage.  This  and  the  next  are  the 
original  forms  described  as  Nauplius,  by  O.  F.  Miiller,  and  believed  at 
that  time  to  be  adult. 

FIGURE  12.  —  Cyclops  tenuicornis.  (After  Claus,  from  Balfour.)  Co- 
pepoda,  Natantia.  Nauplius  ;  second  stage  ;  IV,  maxillae. 


FIGURE  13.  —  Cetochilus septentrional™.  (After  Grobben,  from  Faxon.) 
Copepoda,  Natantia.  Nauplius;  just  hatched;  ventral  view. 

FIGURE  14.  —  Adheres  percarum.  (After  Claus,  from  Faxon.)  Co- 
pepoda, Parasitica.  Larva  at  the  time  it  leaves  the  egg,  with  only  two 
anterior  unbranched  pairs  of  appendages  of  the  typical  nauplius  present. 
Under  the  skin  are  the  rudiments  of  six  pairs  of  appendages  ;  ///,  man- 
dibles; IV,  maxillae;  V,  VI ',  maxillae;  VII,  VIII,  swimming  feet. 

FIGURE  15.  — Balanus  balanoides.  (After  Hoek,  from  Faxon.)  Cirri- 
pedia.  Nauplius. 

FIGURE  16.  —  Lerncediscusporcellance.  (After  F.  Miiller,  from  Faxon.) 
Cirripedia,  Rhizocephala.  Nauplius;  ventral  side;  showing  outline  of 
dorsal  shield. 

FIGURE  17.  —  Sacculina purpurea.  (After  F.  Miiller,  from  Huxley  and 
Balfour.)  Cirripedia,  Rhizocephala. 

FIGURE  18.  —  Cyprus  ovum.  (After  Claus,  from  Faxon.)  Ostraeoda. 
First  larval  (nauplius)  stage,  with  bivalve  shell  and  unbranched  second 
and  third  pairs  of  appendages. 

FIGURE  19.  —  Nebalia  Geoff royi.  (After  Metschnikoff,  from  Faxon.) 
Leptostraca.  Side  view  of  the  so-called  nauplius  stage  of  the  embryo 
within  the  egg.  Rudiments  are  present  of  the  two  pairs  of  antennas  (/. 
//)  and  the  mandibles  (///). 

FIGURE  20.  —  Euphausia.  (After  Metschnikoff,  from  Faxon.)  Schi- 
zopoda.  Nauplius  ;  just  hatched. 

FIGURE  21.  —  Euphausia.  (After  Metschnikoff,  from  Faxon.)  Schi- 
zopoda.  Nauplius  at  a  later  stage;  ventral  view  ;  mt,  metastoma;  IV,  V, 
maxillae  ;  VI,  maxilliped.  In  the  next,  or  Protozoen,  stage  the  append- 
ages (IV,  V,  VI)  are  true  phyllopodiform  feet. 

FIGURE  22 Mysis  ferruginea.    (After  Van  Beneden,  from  Faxon.) 

Schizopoda.  Nauplius-like  embryo ;  side  view.  The  appendages  are 
unsegmented,  and  the  third  pair  is  quite  rudimentary.  A  number  of  later 
metamorphoses  are  undergone  in  the  nauplius  skin,  until  the  full  number 
of  appendages  is  developed. 

FIGURE  23.  —  Peneus.  (After  F.  Miiller,  from  Faxon.)  Decapoda, 
Macroura.  Nauplius;  from  dorsal  side. 

FIGURE  24.  —  Lucifer.  (After  Brooks,  from  Faxon.)  Decapoda,  Ma- 
croura. Ventral  view  of  embryo  artificially  removed  from  the  egg  ;  /  V, 
V,  VI,  buds  representing  the  two  pairs  of  maxillae  and  first  pair  of  max- 
illipeds  of  the  adult. 

FIGURE  25.  —  Palcemon.  (After  Bobretzky,  from  Faxon.)  Decapoda, 
Macroura.  Nauplius  stage  of  embryo  within  the  egg. 

FIGURE  26.  —  Astacus  fluviatilis.  (After  Reichenbach,  from  Faxon.) 
Decapoda,  Macroura.  Nauplius  stage  of  embryo. 

FIGURE  27.  —  Limulus  polyphemus.  (After  Kingsley.)  Xiphosura. 
Ventral  view  of  embryo ;  showing  the  budding  of  the  legs. 

FIGURE  28. — Limulus  polyphemus.  (After  Packard,  from  Balfour.) 
Xiphosura.  Ventral  view  of  embryo  in  the  egg  ;  showing  the  rudiments 
of  six  pairs  of  legs  ;  m,  mouth. 

FIGURE  29. — Limulus  polyphemus.  (After  Packard,  from  Balfour.) 
Xiphosura.  Oblique  side  view  of  embryo,  with  the  mouth  and  rudimen- 
tary limbs  on  the  ventral  plate. 

The  figures  of  embryonic  Limulus  are  introduced  for  comparison. 
They  are  so  different  from  the  nauplius  that  detailed  notice  seems  un- 
necessary. 


Plate  V 


CRUSTACEAN  LARVJE 


OF  THE 

NIVERSITY 


PLATE   VI 


PLATE    VI 
TRIARTHRUS   BECKI   GREEN   (PAGES  199-202) 

FIGURE  1.  — Dorsal  view  of  larva.     X  28. 

FIGURE  2.  — Dorsal  view  of  young  individual,  with  one  free  thoracic 
segment.  (After  Walcott.) 

FIGURE  3.  —  Cephalon  with  antennae  nearly  at  right  angles  to  axis. 
The  thorax  and  pygidium  are  omitted  in  figures  3-6.  The  figures  are 
enlarged  3-5  diameters. 

FIGURE  4.  —  Cephalon  with  antennae  bent  outward  and  backward. 

FIGURE  5.  —  Cephalon  with  slightly  diverging  antennae  directed  for- 
ward, —  the  usual  position  in  the  majority  of  specimens. 

FIGURE  6. — Cephalon  with  antennae  curving  backward  between  the 
eyes. 

FIGURE  7.  —  Dorsal  view ;  showing  antennae  and  crawling  and  swim- 
ming legs.  X  3.  The  legs  on  the  left  side  are  taken  from  a  smaller  speci- 
men and  are  enlarged  6  diameters. 

FIGURE  8.  —  Appendages  attached  to  right  side  of  second  and  third 
thoracic  segments ;  taken  from  another  specimen. 

FIGURE  9.  —  The  same;  with  setae  omitted  from  77,  to  show  details 
of  structure;  ex,  exopodite;  en,  endopodite.  The  setae  are  represented 
on  777.  X  12. 

Utica  Slate,  Ordovician,  near  Rome,  New  York. 


Plate  VI 


7  9 

APPENDAGES  OF  TRIARTHRUS 


OF  THE 

'DIVERSITY 

OF 


\ 


PLATE   VII 


PLATE    VII 
TRIARTHRUS  BECKI   GREEN   (PAGES  206-209) 

FIGURE  1.  —  Ventral  side  of  a  large  specimen  ;  showing,  at  each  side 
of  the  hypostoma,  the  antennules  with  their  single  flagellum  and  strong 
basal  joint ;  also  the  biramous  cephalic  appendages  with  large  gnatho- 
bases ;  the  biramous  thoracic  limbs  with  gnathobases  extending  obliquely 
backward  in  the  axis  of  the  trilobite ;  and  the  hypostoma,  metastoma, 
and  anus,  in  the  median  line.  X  4.  Utica  Slate,  Ordovician,  near  Rome, 
New  York. 


Plate  VII 


Jill 


iffvv: 

Ifili 


VENTRAL  SIDE  OP  TRIARTHRUS 


PLATE   VIII 


PLATE   VIII 
TRIARTHRUS  BECKI   GREEN   (PAGES   205-211) 

FIGURE  1.  —  Diagrammatic  restoration  of  second  thoracic  limb  in 
transverse  section  of  trilobite;  showing  gnathobases  extending  under 
the  axis  toward  the  median  line,  and  the  biramous  limbs  under  the  pleura 
and  beyond  the  carapace. 

FIGURE  2.  —  Diagrammatic  restoration  of  anterior  pair  of  pygidial 
limbs  ;  showing  phyllopodiform  structure. 

FIGURE  3.  —  Diagrammatic  restoration  of  posterior  pair  of  pygidial 
limbs ;  showing  more  strongly  the  primitive  phyllopodous  structure. 

FIGURE  4. —  Dorsal  view  of  second  thoracic  leg,  with  gnathobase.  X  12. 

FIGURES  5,  6,  7.  —  Dorsal  views  of  three  heads;  showing  antennules. 
Their  position  in  figure  7  is  the  most  common. 

FIGURE  8.  —  Portion  of  under  side  of  head  ;  showing  plate-like  gnatho- 
bases of  cephalic  limbs,  posterior  part  of  hypostoma,  the  metastoma,  and 
one  endopodite  on  the  right.  X  4. 

FIGURE  9.  —  Under  side  of  head,  with  gnathobases  and  metastoma 
better  preserved.  X  4. 

FIGURE  10.  —  Under  side  of  head ;  showing  antennules  and  their 
points  of  attachment,  the  four  pairs  of  gnathobases  of  the  other  cephalic 
limbs,  and  the  hypostoma  and  metastoma  in  the  median  line.  X  4. 

FIGURE  11.  —  Diagram;  showing  the  cephalic  appendages  preserved 
in  figures  5,  6,  7,  and  figure  1,  Plate  VII.  1,  shaft  of  antennule  bearing 
a  single  flagellum  ;  £,  coxopodite  of  first  pair  of  biramous  limbs,  or  pos- 
terior antennae ;  #,  third  pair  of  cephalic  limbs,  or  mandibles ;  4,  5, 
gnathobases  of  fourth  and  fifth  pairs  of  cephalic  limbs,  or  maxillaa ;  %, 
hypostoma;  m,  mouth;  met,  metastoma;  s,  setae.  This  figure  is  not  in- 
tended as  a  complete  restoration  of  the  cephalic  appendages,  but  only  as 
a  diagram  for  convenient  reference,  combining  the  characters  preserved 
in  the  specimens  illustrated. 

FIGURE  12. —  Portion  of  the  under  side  of  the  head  and  thorax  of 
Apus.  hy,  hypostoma;  J?,  antennule;  #,  posterior  antenna ;  5,  mandibles; 
4,  5,  maxillae ;  6,  maxilliped ;  7,  first  thoracic  leg  ;  8,  9,  basal  endites  of 
phyllopodiform  legs.  X  3. 


Plate  VIII 


APPENDAGES  OF  TBIAUTHRUS 


PLATE  IX 


PLATE   IX 
TRIARTHRUS   (PAGES   213-219) 

FIGURE  1.  —  Triarthrus  Becki  Green  ;  dorsal  view;  showing  character 
and  extent  of  antennules  and  limbs  beyond  the  carapace.  X  2^. 

FIGURE  2.  —  Triarthrus  Becki  Green;  ventral  view;  showing  entire 
series  of  appendages,  together  with  hypostoma,  metastoma,  and  anal 
opening.  X  2J. 

Utica  Slate,  Ordovician,  near  Rome,  New  York. 


Plate  IX 


TRIARTHRUS 


V 

OF  THE 

I  'DIVERSITY' 

OF 


PLATE  X 


PLATE   X 
TRINUCLEUS   CONCENTRICUS  EATON  (PAGES  222-225) 

FIGURE  1.  —  Cephalon  of  young  individual,  without  genal  spines ; 
showing  ocular  ridges  and  two  rows  of  perforations  around  anterior  and 
lateral  borders.  X  40. 

FIGURE  2.  —  Cephalon  of  younger  individual  before  the  growth  of  the 
perforate  border ;  showing  distinctly  the  clavate  ocular  ridges,  a,  a.  X  40. 

FIGURES.  —  Pygidium  of  young  individual;  showing  the  indistinct 
limitation  of  axis  and  the  elevated  transverse  ridges  of  the  pleura  and 
axis.  X  40. 

FIGURE  4.  —  Thorax  and  pygidium  of  an  entire  specimen  from  which 
the  dorsal  test  has  been  removed  by  weathering,  exposing  below  the 
fringes  of  the  exopodites,  which  entirely  cover  the  pleural  portions.  The 
stronger  lines  ascending  from  the  axis  are  the  main  stems  of  the  exopo- 
dites. The  black  dots  along  the  axis  are  the  fulcra  for  the  attachment 
of  the  limbs.  X  4. 

FIGURE  5.  —  One-half  the  pygidium  with  three  attached  thoracic  seg- 
ments, from  an  entire  specimen,  with  a  portion  of  the  test  removed ; 
showing  the  highly  developed,  lamellose  fringes  of  the  exopodites.  X  H- 

FIGURE  6.  —  The  same ;  lower  side  ;  showing  the  short,  stout,  phyl- 
lopodiform  endopodites,  a,  and  the  long,  slender  exopodites,  &,  bearing 
the  lamellose  branchial  fringes.  In  the  lower  third  of  the  figure  the  ends 
of  the  joints  of  the  separate  endopodites  are  shown  by  the  oblique  ascend- 
ing rows  of  setiferous  nodes.  The  small  ovate  organs,  e,  along  the  side 
are  provisionally  correlated  with  the  exopodites.  A  narrow  striated 
doublure  margins  the  pygidium  and  the  ends  of  the  thoracic 
pleura.  X  11. 

Utica  Slate,  Ordovician,  near  Rome,  New  York. 


Plate  X 


ft    b 


APPENDAGES  OF  TRINUCLEUS 


PLATE   XI 


PLATE    XI 
ATREMATA   (PAGE  243) 

FIGURE  1.  —  Dorsal  valve  of  Iphidea  labradorica  swantonensis  Bil- 
lings. X  3. 

FIGURE  2.  —  Ventral  valve  of  young  specimen.     X  3. 

Cambrian,  near  Georgia,  Vermont. 

FIGURE  3.  —  Apex  of  ventral  valve  of  Glottidia  pyramidata  Stimp- 
son.  X  25. 

FIGURE  4.  —  The  same ;  dorsal  valve ;  showing  more  distinctly  ter- 
minal protegulum.  X  25. 

Recent,  Beaufort,  North  Carolina. 

NEOTREMATA  (PAGE  244) 

FIGURE  5.  —  Upper  valve  of  nepionic  Orbiculoidea  minuta  Hall;  repre- 
senting protegulum,  p,  and  Paterina  stage.  X  25. 

FIGURE  6.  —  More  advanced  condition ;  showing  acquisition  of  dis- 
cinoid  characters.  X  25. 

FIGURE  7.  —  Lower  valve  of  young  specimen  ;  showing  protegulum 
and  open  pedicle-notch.  X  25. 

Devonian,  Marcellus  Shale,  Avon,  New  York. 

FIGURE  8.  —  Accelerated,  discinoid,  dorsal  protegulum  of  Discinisca 
IcEvis  Sowerby  ;  corresponding  to  neanic  stage  of  Orbiculoidea  minuta. 
figure  6.  X  25. 

FIGURE  9.  —  Ventral  protegulum  of  same  species  similarly  modified  ; 
agreeing  with  figure  7.  X  25. 

FIGURE  10. — Lower  valve  of  same  species;  showing  sub-marginal 
position  of  pedicle-opening.  Natural  size. 

Recent,  Callao,  Peru. 

FIGURE  11.  — Lower  valve  of  Schizotreta  tenuilamellata  Hall;  showing 
centripetal  tendency  of  pedicle-opening.  Natural  size. 

Niagara  Group,  Hamilton,  Ontario.  (Pal.  N.  Y.  Extract  from 
vol.  viii,  pi.  iv  E,  fig.  10,  1890.) 

FIGURE  12.  —  Lower  valve  of  Acroihele  subsidua  (after  Linnarsson)  ; 
showing  sub-central  position  of  pedicle-opening.  Natural  size.  Cam- 
brian, Sweden. 


PROTREMATA  (PAGE  245) 

FIGURE  13.  —  Dorsal  protegulum  and  early  nepionic  growth-lines 
of  Plectambonites  segmentina  Angelin.  X  80.  Upper  Silurian,  Gotland, 
Sweden. 

FIGURE  14.  —  Dorsal  protegulum  of  Chonetes  scitulus  Hall.  X  80. 
Hamilton  Group,  Thedford,  Ontario. 

FIGURE  15.  —  Ventral  protegulum  of  Chonetes  granuliferus  Owen ; 
showing  pedicle-notch.  X  80.  Coal  Measures,  Manhattan,  Kansas. 

FIGURE  16.  —  Nepionic  stages  of  ventral  valve  of  Orthoihetes  elegans 
Bouchard.  X  25.  (Compare  with  figure  12  of  Acrothele.)  Devonian, 
Ferques,  France. 

FIGURE  17.  —  Nepionic  stages  of  Stropheodonta  perplana  Conrad ; 
showing  pedicle  perforation,  deltidium,  and  hinge-area.  X  25.  Hamil- 
ton Group,  Falls  of  the  Ohio. 

FIGURE  18. — Ventral  nepionic  stage  oiLeptcena  rhomboidalis  Wilckens. 
X25. 

FIGURE  19.  —  Profile  of  the  same.     X  25. 

Lower  Helderberg  Group,  Albany  county,  New  York. 

FIGURE  20.  —  Hinge  of  a  specimen  2  mm.  in  length  ;  showing  deltid- 
ium and  hinge-area. 

FIGURE  21.  —  Ventral  view  of  specimen  having  same  dimensions ; 
showing  nepionic  and  neanic  stages,  and  relative  proportions  of  pedicle- 
opening  and  shell  at  this  stage.  Niagara  Group,  Waldron,  Indiana. 

Figures  20  and  21  are  taken  from  "Development  of  Some  Silurian 
Brachiopoda,"  Mem.  N.  Y.  State  Museum,  vol.  i,  no.  1,  pi.  ii,  figures  2, 12, 
1889. 

TELOTREMATA   (PAGES   245,   246) 

FIGURE  22.  —  Ventral  view  of  young  Kraussina  (Megerlina)  Lamarcki- 
ana  Davidson  ;  showing  protegulum  and  early  nepionic  stages.  X  80. 

FIGURE  23.  —  Dorsal  view  of  same;  showing  dorsal  protegulum  and 
pedicle-opening  in  ventral  valve.  X  80. 

Recent,  Port  Jackson,  Australia. 

FIGURE  24.  —  Dorsal  view  of  beaks  of  young  Terebratulina  septen- 
trionalis  Couthouy;  showing  dorsal  protegulum  and  pedicle-opening  in 
ventral  valve.  X  80.  Recent,  Eastport,  Maine. 

FIGURES  25-28.  —  Diagrammatic  representation  of  ventral  areas  ; 
showing  progressive  development  of  deltidial  plates. 

Figure  25  is  without  plates,  as  in  ventral  area  of  figure  23.  Figure  26 
shows  two  triangular  plates,  which  unite  by  symphysis  in  figure  27, 
making  an  elongate  pedicle-opening.  In  figure  28  the  pedicle  perforation 
is  sub-circular  and  truncates  ventral  beak.  This  series  corresponds 
essentially  with  that  shown  in  Rhynchotreta  cuneata  Dalman,  in  "  Develop- 
ment of  Some  Silurian  Brachiopoda,"  loc.  ciV.,  pi.  iv,  figures  16-22. 


Plate  XI 


STAGES  OF  GROWTH  OF  BRACHIOPODA 


Vv~          "v  /- 

OF  THE 

UNIVERSITY 


PLATE  XII 


PLATE   XII 

DEVELOPMENT  AND   CLASSIFICATION   OF 
BRACHIOPODA. 

GLOTTIDIA  ALBIDA  HINDS   (PAGE  269) 

FIGURE  1.  — Nepionic  shell;   Obolella  stage.     X  36. 
FIGURE  2.  —  Neanic  stage ;  showing  anterior  growth  producing  Lin- 
gula-like  form.     X  16. 

FIGURE  3.  — Ephebic  stage.     X  f- 


ORBICULOIDEA  MINUTA  HALL   (PAGE  269) 

FIGURE  4.  —  Nepionic  shell;  Palermo,  stage.     X  36. 

FIGURE  5.  —  Neanic  stage ;  first  holoperipheral  growth,     x  16. 

FIGURE  6.  — Ephebic  stage.     X  10. 


LEPT^NA  RHOMBOIDALIS   WILCKENS   (PAGES  268,  269) 

FIGURE  7.  —  Nepionic  stage,  with  short  hinge.     X  36. 
FIGURE  8.  —  Early  neanic  stage,  with  radiating  strise.     X  10. 
FIGURE  9.  —  Ephebic  stage.     X  f  • 

TEREBRATULINA    SEPTENTRIONALIS   COUTHOUY 
(PAGES  268,  269) 

FIGURE  10.  —  Nepionic  stage,  with  open  delthyrium.     X  26. 
FIGURE  11.  — Early  neanic  stage,  with  radiating  strise.    X  16.  (After 
Morse.) 

FIGURE  12.  —  Ephebic  stage,     x  f .     (After  Davidson.) 


10 


11 


STAGES  OF  GROWTH  OP  BRACIIIOPODA 


PLATE   XIII 


PLATE   XIII 
MORPHOGENY  OF  MAGELLANIIN^E   (PAGES  287-289) 

The  figures  in  the  left-hand  column,  A-H,  represent  the  stages  in  the 
ontogeny  of  the  brachial  supports  in  Magellania,  one  of  the  highest 
genera  of  the  family  Terebratellidse.  In  the  right-hand  column  are 
shown  the  adult,  permanent,  generic  structures,  corresponding  to  the 
stages  of  Magellania. 

Terebratella  passes  through  all  the  stages  from  A  to  G,  Magasella  from 
A  to  F,  and  so  on,  as  far  as  known,  for  each  lower  genus. 

All  figures  are  drawn  of  approximately  the  same  length,  to  facilitate 
comparison ;  in  consequence,  the  younger  stages  are  much  enlarged. 

FIGURE  A.  —  Early  larval  brachiopod,  without  calcified  brachial  sup- 
ports, but  with  circlet  of  tentacles  on  lophophore.  The  gwyniform  stage. 

FIGURE  Ai. —  Gwynia  capsula  Jeffreys;  a  morphic  equivalent  of  larval 
stage,  figure  A. 

FIGURE  B.  —  Later  stage  of  A  ;  showing  growth  of  septum  and  con- 
sequent introversion  of  edge  of  lophophore.  Cistelliform  stage. 

FIGURE  Bi.  —  Cistella  neapolitana  Scacchi ;  showing  calcification  of 
loop  attached  to  septum,  and  other  adult  features.  Morphic  equivalent 
of  stage  B  of  Magellania. 

FIGURE  C.  — Third  stage  of  Magellania,  with  small  ring  on  septum. 
Bouchardiform  stage. 

FIGURE  Ca.  —  Side  view  of  same. 

FIGURE  Ci.  —  Bouchardia  rosea  Mawe;  adult;  showing  ring  on  sep- 
tum as  in  C. 

FIGURE  D.  —  Megerliniform  stage  of  Magellania. 

FIGURE  Da.  —  Side  view  ;  showing  growth  of  descending  branches  as 
prongs  on  side  of  septum. 

FIGURE  Di.  — Megerlina  Lamarckiana  Davidson;  adult  form  of  brach- 
ial supports. 

FIGURE  E.  —  M agadiform  stage  of  Magellania;  showing  completion  of 
descending  branches. 

FIGURE  Ei.  — Magas  pumilus  Sowerby,  the  Cretaceous  prototype  of 
this  structure. 

FIGURE  F.  —  Magaselliform  stage  ;  showing  union  of  descending  and 
ascending  branches. 

FIGURE  Fi.  —  Magasella  Cumingi  Davidson. 

FIGURE  G.  —  Terabratelliform  stage ;  representing  the  finished  type  of 
structure  in  Terebratella  dorsata  Gmelin. 

FIGURE  Gi.  —  Terebratella  rubicunda  Sowerby.  Morphically  equiva- 
lent to  G,  but  showing  more  mature  features. 

FIGURE  H.  — Final  stage  of  Magellania  venosa  Solander,  produced  by 
resorption  of  the  septum  and  connecting  bands  of  the  terebratelliform  stage. 

FIGURE  Hi.  —  Magellania  flavescens  Lamarck. 


Plate  XIII 


I  ^ 

<v.^> 

PARALLELISM  IN  BRACHIOPODA  (Magellania  Series) 


PLATE   XIV 


PLATE  XIV 
ONTOGENY  AND  PHYLOGENY  OF  THE  TEREBRATELLID^E 

Figures  represent  brachial  supports  in  various  stages  of  growth  in  dif- 
ferent genera  and  species.  All  are  drawn  of  approximately  the  same 
length  to  facilitate  comparison,  so  that  in  general  the  younger  stages  are 
much  enlarged.  Vertical  rows  of  figures  connected  by  dotted  lines  indi- 
cate ontogeny  as  far  as  known.  Horizontal  rows  indicate  the  same 
growth-stages  of  higher  forms  and  the  adult  structure  in  genera  repre- 
senting these  stages.  All  figures  are  of  recent  species  unless  otherwise 
stated. 

MORPHOGENY   OF   MEGATHYRIN^E     (Page  300) 

FIGURE  A.  —  Early  larval  brachiopod  without  calcified  brachial  sup- 
ports, but  with  circlet  of  centripetal  tentacles  on  the  lophophore.  The 
gwyniform  stage. 

FIGURE  Aa.  —  Gwynia  capsula  Jeffreys  ;  a  morphic  equivalent  of  larval 
stage,  figure  A. 

FIGURE  B.  —  Later  immature  stage  of  A ;  showing  growth  of  septum, 
and  consequent  introversion  of  edge  of  lophophore.  Early  cistelliform 
stage. 

FIGURE  Ba. —  Zellania  Hasina  Moore,  from  the  Lias;  a  morphic 
equivalent  of  figure  B. 

FIGURE  Bi.  —  Cistella  neapolitana  Scacchi ;  showing  calcification  of 
loop  attached  to  septum,  and  other  adult  features. 

FIGURE  62.  —  Megathyris  decollata  Chemnitz ;  adult  shell ;  showing 
advance  over  Cistella  in  the  two  lateral  septa,  thus  increasing  the  length 
of  the  loop. 

MORPHOGENY  OF  DALLININ^E  (Pages  295-299) 
Ontogeny  of  Macandrevia  cranium  Miiller 

FIGURE  A.  —  Gwyniform  stage. 

FIGURE  B.  —  Cistelliform  stage. 

FIGURE  Ci.  —  Platidiform  stage ;  showing  union  of  descending  lamellae 
with  dorsal  septum. 

FIGURE  Ci'.  —  Side  view  of  septum  of  preceding. 

FIGURE  Di.  —  More  advanced  platidiform  stage  ;  showing  growth  of 
ascending  branches,  or  secondary  loop. 

FIGURE  Di'.  —  Side  view  of  same. 

FIGURE  Ei.  —  Ismeniform  stage,  with  ascending  branches  still  attached 
to  septum. 

FIGURE  Fi. — Miildfeldtiform  stage;  showing  holes  in  ascending 
lamellae  as  in  Muhlfeldtia,  figure  Fa. 


FIGURE  Gi.  —  Terebrataliform  stage,  with  ascending  branches  attached 
to  ends  of  descending  branches. 

FIGURE  Hi.  —  Final  adult  condition  of  Macandrevia  cranium  Miiller; 
showing  absorption  of  septum  and  connecting  bands  from  descending 
branches. 

FIGURE  Ha.  —  Adult  stage  of  Macandrevia  tenera  Jeffreys. 

Ontogeny  of  Dallina  septigera  Loven 

FIGURES  A,  B,  Ci.  —  Stages  of  growth  common  to  all  the  genera 
of  Dallininge. 

FIGURE  Da.  —  Late  platidiform  stage. 

FIGURE  Ea.  —  Ismeniform  stage. 

FIGURE  F2.  —  Muhlfeldtiform  stage. 

FIGURE  Ga.  —  Terebrataliform  stage. 

FIGURE  Hs.  —  Adult  Dallina  septigera  Love'n. 

FIGURE  H4.  —  Dallina  Rapliaelis  Dall. 

FIGURE  H5.  — Dallina  floridana  Pourtales. 

FIGURE  He.  —  Dallina  Grayi  Davidson. 

FIGURES  Ca,  Ds,  Es,  Fs.— Ontogeny  of  Muhlfeldtiasan  guinea  Chemnitz. 

FIGURE  Gs.  —  Terebratalia  coreanica  Adams  and  Reeve;  adult  structure. 

FIGURE  F4.  —  Muhlfeldtiform  stage  of  Laqueus  californicus  Koch. 

FIGURE  G4.  —  Adult  Laqueus  californicus  Koch ;  showing  connecting 
bands  from  ascending  and  descending  lamellae. 

FIGURE  Gs.  —  Laqueus  pictus  Chemnitz. 

FIGURE  Ge.  —  Trigonosemus  elegans  Koenig;  a  Cretaceous  representa- 
tive of  the  Terebratalia  structure. 

FIGURE  E4. — Ismenia  furcata  Sowerby;  Jurassic. 

FIGURE  E5. — Ismenia  Buckmani  Moore ;  Jurassic. 

FIGURE  Cs.  —  Platidiform  stage  of  Dallina  Jloridana  Pourtales  V 

FIGURE  C4.  —  Adult  Platidia  anomioides  Scacchi. 


MORPHOGENY    OF    MAGELLANIINvE    (PagCS  293-295) 

Ontogeny  of  Terebratella  dorsata  Gmelin,  and  Magellania  venosa  Solander 

FIGURE  A.  —  Gwyniform  stage. 

FIGURE  B.  —  Cistelliform  stage. 

FIGURE  Ca.  —  Boucnardiform  stage,  with  small  ring  on  septum. 

FIGURE  Ca'.  —  Side  view  of  same. 

FIGURE  Da. — Megerliniform  stage;  showing  growth  of  ring,  or 
ascending  branches. 

FIGURE  Da'.  —  Side  view;  showing  growth  of  descending  branches 
as  prongs  on  side  of  septum. 

FIGURE  Ea.  —  Magadiform  stage  ;  showing  completion  of  descending 
branches. 

FIGURE  Fa. —  Ufagaselliform  stage;  showing  union  of  descending  and 
ascending  branches. 

FIGURE  Ga.  —  Terebratelliform  stage ;  representing  the  finished  type 
of  structure  in  Terebratella  dorsata  Gmelin. 

FIGURE  Ha. — Final  stage  of  Magellania  venosa  Solander;  produced 
by  resorption  of  the  septum  and  connecting  bands  of  the  terebratelliform 
stage. 


MAGE1LL  ANMIN 


0 


ONTOGENY  AND  PHYLOGE 


Plate  XIV 


D  ALLVNMN  AC 

w*      T^\oT\3io.i(v.a. 


f 

.5 

(2 


o 


OF  THE  TEREBRA.TELLID.E 


FIGURE  Hb.  —  Magellania  Wyvillii  Davidson. 

FIGURE  lid.  —  Magellania  flavescens  Lamarck. 

FIGURE  He.  — Magellania  kerguelenensis  Davidson. 

FIGURES  E6,  F6,  Gb,  He.  —  Ontogeny  of  Magellania  (Neoihyris)  len- 
ticularis  Deshayes,  from  the  magadiform  to  the  magellaniform  stages. 

FIGURES  EC,  Fc,  Gc.  —  The  magadiform,  magaselliform,  and  terebratelli- 
form  stages  of  Terebralella  cruenta  Dillwyn. 

FIGURES  Ed,  Fd,  Gd.  —  The  same  stages  in  Terebratella  rubicunda 
G.  B.  Sowerby. 

FIGURE  E.  —  Magas  pumilus  Sowerby;  the  Cretaceous  prototype  of 
this  structure. 

FIGURES  E/,  Fe.  — The  magadiform  and  magaselliform  stages  of  Mag- 
asella  Cumingi  Davidson. 

FIGURE  t>b.  —  Megerlina  Lamarckiana  Davidson ;  adult  form  of 
brachial  supports. 

FIGURE  Cb.  —  Kraussina  rubra  Pallas ;  adult ;  showing  septum  and 
two  branches. 

FIGURE  Cc.  —  Kraussina  pisum  Valenciennes  apud  Lamarck. 

FIGURE  Cc?. — Bouchardia  rosea  Mawe;  adult;  showing  thickened 
cardinal  process,  hinge-plate,  and  ring  on  the  septum  as  in  Ca. 


PLATE   XV 


PLATE   XV 


CRANIA  SILURIANA  HALL   (PAGE  317) 

FIGURE  1.  — The  youngest  individual  observed  ;  having  a  height  of 
1  mm.  and  a  width  across  the  base  of  1.5  mm.  The  elevation  of  the 
shell  is  in  strong  contrast  to  that  of  the  mature  form. 

FIGURE  2.  — ••  A  mature  individual  attached  to  a  shell  of  Platystoma. 
(2Sth  Kept.  N.  Y.  State  Mm.  Nat.  Hist.,  pi.  21,  fig.  5.) 

DALMANELLA  ELEGANTULA  DALMAN   (PACKS  317-321) 
See  Plate  XXII 

FIGURE  3.  —  Dorsal  view  of  the  youngest  embryo  observed,  its  length 
being  .5  mm.,  its  width,  .75  mm.  The  median  sinus  has  already  formed, 
and  three  pairs  of  plications  have  appeared,  of  which  the  middle  pair  is 
evidently  the  oldest. 

FIGURE  3a.  —Outline  profile  of  the  same;  showing  the  slightly  greater 
convexity  of  the  ventral  valve. 

FIGURE  4.  —  A  larger  example  viewed  from  the  dorsal  side,  its  length 
being  1  mm.,  its  width  1.5  mm.  The  plications  have  now  increased  to 
the  number  of  six  pairs,  one  of  which  has  appeared  between  the  median 
plications  seen  in  figure  3.  , 

FIGURE  5.  — Ventral  view  of  an  individual  in  about  the  same  stage  of 
development ;  showing  a  strong  median  plication  corresponding  to  the 
dorsal  sinus,  and  five  pairs  of  lateral  plications. 

FIGURE  6.  —  Cardinal  view  of  an  individual  in  the  growth-stage  repre- 
sented by  figure  5.  The  valves  have  nearly  the  same  convexity,  while 
the  width  of  the  cardinal  area  and  the  size  of  the  pedicle-passage  are  the 
same  for  each.  The  latter  is  seen  to  be  quite  unobstructed  and  without 
further  differentiation  than  a  slight  thickening  of  the  margins. 

FIGURE  Qa.  —  Outline  profile  of  the  same. 

FIGURE  7.  —  Cardinal  view  of  an  individual  which  has  reached  a  size 
of  3  X  3  mm.  Here  a  change  is  apparent  in  the  development  of  the  car- 
dinal area  and  foramen  of  the  valves.  The  primary  indication  of  the 
callosity  or  cardinal  process  is  in  the  apex  of  the  dorsal  opening.  The 
difference  in  the  convexity  of  the  valves  has  also  noticeably  increased. 

FIGURE  7a.  —  Outline  profile  of  the  same. 

FIGURE  8.  —  Cardinal  view  at  a  size  of  5  X  5  mm.  The  ventral  beak 
has  become  strongly  incurved,  and  the  cardinal  process  is  now  sub-divided 


into  three  parts.  The  cross-lines,  representing  the  natural  size  of  the 
specimen,  are  too  short. 

FIGURE  8a.  —  Outline  profile  of  the  same. 

FIGURE  9.  —  Cardinal  view  at  a  size  of  12  X  11  nun.  The  ventral 
valve  and  area  have  become  greatly  curved,  and  the  dorsal  aperture  is- 
quite  filled  by  the  tripartite  cardinal  process. 

FIGURE  9a.  —  Outline  profile  of  the  same. 

FIGURE  10.  —  Cardinal  view  of  a  large  adult;  size  18  X  18.5  mm. 
The  areas  are  closely  appressed,  and  the  dorsal  aperture  is  wholly  filled 
by  the  cardinal  process,  the  central  portion  of  which  extends  into  the 
aperture  of  the  other  valve. 

FIGURE  10a.  —  Outline  profile  of  the  same. 

FIGURE  11.  —  Cardinal  view  of  a  normal  adult;  natural  size. 

FIGURE  lla.  —  Profile  of  the  same.     (op.  cit.,  pi.  21,  figs.  17,  14.) 

FIGURE  12.  —  Dorsal  view  of  a  small  adult. 

FIGURE  12a.  — Ventral  view  of  the  same.     (op.  cit.,  figs.  12,  11.) 

RHIPIDOMELLA   IIYBRIDA   SOWERBY   (PAGES  321,  322) 

FIGURE  13.  —  Cardinal  view  of  a  very  young  individual  having  a  length 
of  1  mm.  and  a  width  of  1.5  mm.  The  valves  are  nearly  equi-convex,  the 
area  and  apertures  as  in  the  earlier  stages  of  D.  elegantula. 

FIGURE  13«.  —  Outline  profile  of  the  same. 

FIGURE  14.  —  Cardinal  view  of  a  somewhat  gibbous  example  measur- 
ing 10  X  8  mm.  The  relatively  short  areas  are  about  equally  developed, 
and  the  cardinal  callosity  of  the  dorsal  valve  has  already  filled  the  dorsal 
aperture. 

FIGURE  14a.  —  Outline  profile  of  the  same. 

FIGURE  15.  —  Cardinal  view  of  a  normal  adult  12  X  10  mm. ;  showing 
the  short  area  and  the  projection  of  the  cardinal  process  into  the  ventral 
aperture. 

FIGURE  15a.  —  Outline  profile ;  showing  the  incurvature  of  the  areas. 

FIGURE  16.  —  Dorsal  view  of  a  large  adult,     (op.  cit.,  fig.  20.) 

FIGURE  17.  —  Profile  of  a  normal  adult,     (op.  cit.,  fig.  21.) 

FIGURE  18.  —  Dorsal  view  of  a  small,  gibbous  example  ;  showing- 
strong  varices.  Enlarged  to  two  diameters. 

FIGURE  ISa.  — Profile  of  the  same. 


3a 


9a 


10  a 


12  a 


lla 


13a 


.01 


14a 


15a 


18  a 


STAGES  OF  GROWTH  IN  SILURIAN  BRACIIIOPODA 


PLATE   XVI 


PLATE   XVI 
LEPT^ENA  RHOMBOIDALIS   WILCKENS  (PAGES  322-327) 

FIGURE  1.  —  Ventral  view  of  the  youngest  shell  observed,  its  length 
being  1.25  mm.  The  aperture  of  the  embryonal  pedicle-sheath  is  very 
conspicuous,  and  its  margins  are  very  thick.  The  surface  shows  a  faint 
median  depression,  indications  of  two  concentric  growth-lines,  and  out- 
side the  latter  of  these,  obscure  traces  of  plications. 

FIGURE  la.  —  Outline  profile  of  the  same;  showing  the  prominence  of 
the  sheath. 

FIGURE  2.  —Ventral  view  of  an  individual  with  a  length  of  2  mm. 
The  aperture  of  the  pedicle-sheath  is  relatively  somewhat  diminished  in 
size,  its  margins  have  become  thinner,  and  the  radiating  plications  nu- 
merous and  sharply  defined. 

FIGURE  2a. — Outline  profile  of  the  same;  indicating  diminution  in 
the  prominence  of  the  sheath. 

FIGURE  3.. —  Ventral  view  of  an  individual  having  a  length  of  4  mm. ; 
showing  the  increase  in  the  number  of  plications,  the  appearance  of  nu- 
merous concentric  undulations  and  strise,  and  the  narrowing  pedicle- 
aperture. 

FIGURE  3a.  —  Outline  profile  of  the  same ;  showing  the  concentric 
undulations  and  the  diminishing  pedicle-sheath. 

FIGURE  4.  —  Ventral  view  of  a  normal  adult,  having  a  length  of 
28  mm. ;  showing  the  characters  of  maturity.  Natural  size. 

FIGURE  4«.  —  Profile  of  another  individual  of  full  growth  ;  showing 
the  anterior  geniculation  and  the  length  of  the  anterior  slope,  or  curtain. 
(28th  Kept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi.  22,  figs.  6,  7.) 

FIGURE  5.  —  Cardinal  view  of  the  specimen  represented  by  figure  1 . 

FIGURE  6.  — Similar  view  of  the  specimen  represented  by  figure  2. 

FIGURE  7.  —  Similar  view  of  an  individual  2.65  mm.  in  length. 

FIGURE  8.  — •  Similar  view  of  the  specimen  represented  by  figure  3. 

FIGURE  9.  —  Similar  view  of  an  individual  9  mm.  in  length. 
These  cardinal  views  are  drawn  with  the  same  degree  of  enlargement, 
and  show  the  gradual  diminution  in  height  and  in  diameter  of  aperture 
in  the  pedicle-sheath,  and  the  increasing  development   of  the  grooved 
callosity  on  the  dorsal  valve. 

FIGURE  10. — Cardinal  view  of  a  normal  adult;  showing  the  great 
size  of  the  grooved  callosity,  and  the  csecal  opening,  representing  the 
atrophied  pedicle-sheath.  Natural  size.  (op.  cit.,  fig.  10.) 


FIGURE  11.  —  The  cardinal  area  represented  in  figure  5  (length 
1.25  ram.),  still  further  enlarged  ;  showing  the  broad,  prominent,  exsert 
sheath,  embracing,  at  its  base,  the  faint,  grooved  dorsal  callosity. 

FIGURE  12.  —  The  cardinal  area  shown  in  figure  6  (length  2  mm.), 
enlarged  to  the  size  of  figure  11;  showing  the  depression  of  the  sheath, 
the  narrowing  of  the  cardinal  area,  and  the  increasing  aperture  between 
the  sheath  and  the  callosity. 

FIGURE  13.  —  The  pedicle-area  of  a  mature  individual.  The  sheath 
is  now  wholly  absorbed,  the  sole  trace  of  it  being  seen  in  the  csecal 
foramen,  surrounded  by  the  urnbonal  portion  of  the  shell.  The  callosity 
is  strongly  developed,  but  not  sufficiently  to  close  the  gap  between  it  arid 
the  opposite  valve,  thus  leaving  a  passage  between  the  valves  and  along 
the  dorsal  groove.  X  2. 

ORTHOTHETES   SUBPLANUS  CONRAD   (PAGES  327-330) 
See  Plate  XXII 

FIGURE  14.  —  Ventral  view  of  the  smallest  individual  observed ;  hav- 
ing a  length  of  2.25  mm.  Both  primary  and  secondary  plications  and 
concentric  growth-lines  have  already  appeared;  indicating  the  very  early 
assumption  of  these  characters. 

FIGURE  14a.  —  Outline  profile  of  the  same  ;  showing  the  convexity  of 
the  valves. 

FIGURE  15.  —  A  normal  adult ;  dorsal  view. 

FIGURE  15a.  —  The  same  in  profile,     (op.  c«7.,  pi.  21,  figs.  30,  31.) 

FIGURE  16.  —  Cardinal  view  of  specimen  somewhat  larger  than  that 
represented  in  figure  14.  The  ventral  valve  bears  a  small  pedicle-sheath, 
the  dorsal,  the  inception  of  a  cardinal  process  or  callosity,  while  between 
the  two  is  a  broad  opening  which  serves  to  indicate  that  at  this  early 
age  the  pedicle-sheath  had  ceased  its  function. 

FIGURE  17.  —  Cardinal  view  of  an  individual  slightly  below  normal 
full  growth,  but  with  essentially  mature  characters. 

FIGURE  18.  —  The  pedicle-area  of  the  specimen  represented  in  fig- 
ure 16. 

FIGURE  19.  —  Pedicle-area  of  a  shell  having  a  length  of  4  mm.  At 
this  stage  of  growth  the  sheath  has  relatively  diminished  in  size,  while 
the  dorsal  callosity  has  increased  and  shows  a  median  groove  on  its  inner 
edge.  Deltidial  plates  have  also  begun  to  develop  along  the  margins  of 
the  ventral  aperture. 

FIGURE  20.  —  Pedicle-area  of  the  specimen  represented  in  figure  17. 
The  sheath  is  now  atrophied  and  altogether  obsolete,  the  dorsal  callosity 
is  very  large,  nearly  filling  the  aperture  between  the  valves,  and  the  del- 
tidial  plates  have  attained  the  maximum  development  observed  in  the 
Strophomenidae . 

The  last  three  figures  have  the  same  degree  of  enlargement. 


2a 


15  a 


(\ 


STAGES  OF  GROWTH  IN  SILURIAN  BRACHIOPODA. 


PLATE   XVII 


PLATE   XVII 
STROPHONELLA   STRIATA   HALL  (PAGES  330-334) 

FIGURE  1.  — Ventral  view  of  the  incipient  shell  of  the  series  ;  length 
2.25  mm. ;  showing  the  opening  of  the  pedicle-sheath,  and  the  primary 
surface  plications. 

FIGURE  la.  —  Outline  profile  of  the  same ;  showing  the  complete  con- 
vexity of  the  ventral  valve,  and  essentially  conformable  concavity  of  the 
dorsal  valve. 

FIGURE  2.  —  Ventral  valve  of  a  normal  adult ;  showing  the  umbonal 
convexity  of  the  shell  and  general  concavity  over  the  pallial  region. 
(28*7*  Rept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi.  23,  fig.  1.) 

FIGURE  2a.  —  Outline  profile  of  the  same ;  showing  the  reversal  in 
convexity  from  the  embryonic  condition. 

FIGURE  3.  —  Pedicle-area  of  the  specimen  represented  in  figure  1 ; 
length  2.25  mm.  ;  showing  the  well-developed,  slightly  exsert  sheath 
and  the  obscure  dorsal  callosity. 

FIGURE  4. —  Pedicle-area  of  an  individual  2.5  mm.  in  length,  in 
which  the  sheath  is  extravagantly  exsert. 

FIGURE  5.  —  Pedicle-area  in  an  example  about  6  mm.  in  length. 

FIGURE  6.  —  Pedicle-area,  when  a  length  of  8  mm.  has  been  attained. 

FIGURE  7.  —  Pedicle-area  in  a  shell  measuring  13  mm.  in  length. 

FIGURE  8.  —  Pedicle-area  in  a  normal  adult  measuring  17  mm.  in 
length. 

Figures  3-8  have  been  drawn  to  the  same  scale,  and  show  the  succes- 
sive phases  in  the  development  of  the  pedicle  characters.  The  sheath 
ceases  its  function  as  a  pedicle-passage  before  maturity  is  attained, 
though  retaining  its  relative  size  ;  while  the  dorsal  callosity,  which  in  the 
earlier  stages  is  grooved  and  largely  enveloped  by  the  sheath,  is  eventu- 
ally separated  from  the  sheath  by  a  narrow  aperture,  and  its  surface 
becomes  uninterrupted. 

MIMULUS   WALDRONENSIS  MILLER  AND  DYER  (PAGES  334,  335) 

FIGURE  9.  —  Dorsal  view  of  a  young  individual  having  a  length  of 
3  mm.  The  shell  is  nearly  symmetrical,  and  shows  an  open  triangular 
deltidium,  ending  in  a  sub-circular  apical  foramen. 

FIGURE  9a.  —  Ventral  view  of  the  same  ;  showing  the  apical  foramen. 

FIGURE  96.  —  Outline  profile  of  the  same. 


FIGURK  10.  —  Cardinal  view  of  an  adult  specimen  ;  showing  the  asym- 
metrical shell. 

FIGURE  10a.  —  Dorsal  view  of  the  same. 

FIGURE  106. — Ventral  view  of  the  same,  (llth  Ann.  Kept.  State 
Geol.  Ind.,  pi.  27,  figs.  21,  19,  20.) 


DICTYONELLA   RETICULATA  HALL  (PAGES  335-337) 

FIGURE  11.  —  A  young  individual  having  a  length  and  width  of 
3  mm.;  showing  the  sub-circular  outline  and  undefined  median  fold. 

FIGURE  12.  —  Axial  section  of  a  larger,  but  immature  form;  indicat- 
ing the  character  of  the  articulation,  and  showing  the  internal  ventral 
plate  and  dorsal  septum. 

FIGURE  13.  —  Dorsal  view  of  an  adult  shell.     X  2. 

FIGURE  13  a.  — Cardinal  view  of  the  same;  showing  the  bare  umbonal 
area  and  the  lines  of  lateral  attachment  of  the  internal  plate.  X  2. 

(Figures  13,  13a,  from  the  28th  Kept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi. 
26,  figs.  53,  54.) 


ANASTROPHIA   INTERNASCENS   HALL  (PAGES  337-339) 

FIGURE  14.  —  Ventral  view  of  the  youngest  specimen  observed  ;  length 
2  mm. 

FIGURE  14«.  — Outline  profile  of  the  same;  showing  the  elevation  of 
the  ventral  beak  and  cardinal  area. 

FIGURE  15.  —  Dorsal  view  of  a  large  adult. 

FIGURE  16.  —  Ventral  view  of  an  average  adult ;  showing  the  over- 
lapping dorsal  valve. 

FIGURE  16a.  —  Profile  of  the  same  ;  showing  the  relative  convexity  of 
the  valves. 

(Figures  15-16a  from  the  28th  Kept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi.  26, 
figs.  49,  45,  44.) 

CAMAROTCECHIA   INDIANENSLS   HALL  (PAGES  346-351) 

FIGURE  17.  —  The  earliest  stage  of  growth  observed,  the  shell  having 
a  length  of  .65  mm.  The  characters  are  essentially  primitive  ;  the  sur- 
face is  without  plications,  the  foramen  triangular  and  devoid  of  plates  or 
even  marginal  thickening. 

FIGURE  17a.  —  Outline  profile  of  the  same  ;  showing  the  elevation  of 
the  ventral  umbo. 

FIGURE  18.  —  A  later  stage  of  growth  in  which  the  shell  has  a  length 
of  1.5  mm.     With  the  formation  of  the  first  growth-line,  a  number  of 
faint  plications  have  appeared,  and  the  margins  of  the  foramen  have" 
become  thickened. 

FIGURE  18a.  — Outline  profile  of  the  same. 


FIGURE  19.  —  A  young  example  with  a  length  of  6  mm. ;  showing  the 
inception  of  the  median  fold,  (llth  Ann.  Rep.  Geol.  Surv.  Ind.,  pi.  27, 
fig.  6.) 

FIGURE  20.  —  Dorsal  view  of  a  larger  example,  having  two  plications 
on  the  fold,  and  abnormal  in  the  absence  of  all  lateral  plications. 
Natural  size.  (op.  cit.,  fig.  5.) 

FIGURE  21.  —  Dorsal  view  of  a  small,  essentially  mature  example, 
with  two  plications  on  the  fold.  (op.  cit.,  fig.  4.) 

FIGURE  22.  —  Dorsal  view  of  an  adult  with  three  plications  on  the  fold. 
(2Sth  Rept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi.  26,  fig.  13.) 

FIGURE  23. —  Front  view  of  an  adult  with  four  plications  on  the  fold. 
(op.  cit.,  fig.  22.) 

FIGURE  24.  —  Dorsal  view  of  a  similar  individual. 

FIGURE  24a.  —  Profile  of  the  same.     (op.  cit.,  figs.  15,  19.) 

FIGURE  25.  —  An  enlargement  of  the  cardinal  area  in  an  individual 
with  a  length  of  1.5  mm. ;  showing  essentially  the  primitive  characters 
seen  in  figure  17. 

FIGURE  25a.  — Outline  profile  of  the  same. 

FIGURE  26.  —  The  condition  of  the  ventral  cardinal  area  and  foramen 
in  an  individual  3.5  mm.  in  length.  The  margins  of  the  foramen  are 
thickened  by  the  inception  of  the  deltidial  plates,  and  the  aperture  is  seen 
to  encroach  upon  the  apical  portion  of  the  shell. 

FIGURE  26a.  —  Outline  profile  of  the  same. 

FIGURE  27.  —  The  cardinal  features  in  an  individual  of  7  mm.  length. 
The  advance  upon  the  last  stage  is  chiefly  in  rapid  development  of  the 
deltidial  plates,  which  have  narrowed  the  opening,  slightly  constricting' 
it  near  the  now  incurved  umbo. 

FIGURE  27a.  —  Outline  profile  of  the  same. 

FIGURE  28.  —  The  cardinal  features  in  an  adult,  having  a  length  of 
10  mm.  The  deltidial  plates  have  united  at  their  base,  forming  an  elon- 
gate oval  aperture,  encroaching  upon  the  umbo.  The  species  does  not 
pass  this  stage  of  development. 

FIGURE  28a.  — Outline  profile  of  the  same. 


,'flb  10 


lOa 


31 


12 


140, 


17a 


-\ 


20 


18  18a 


16  a 


22. 


23 


24a 


27 


28a 


25  25a 

A      A 


26  26a 


/A 


21  a 


STAGES  OF  GROWTH  IN  SILURIAN  BRACIIIOPODA 


PLATE   XVIII 


PLATE   XVIII 
CAMAROTCECHIA   WHITII   HALL   (PAGES  344-346) 

FIGURE  1.  —  The  earliest  observed  stage  of  growth,  the  shell  measur- 
ing 2.75  mrn.  in  length  by  2  mm.  in  width.  The  deltidial  plates  have 
already  begun  to  form  along  the  edges  of  the  triangular  foramen,  and 
four  pairs  of  plications  are  visible  on  the  dorsal  valve. 

FIGURE  la.  —  Outline  profile  of  the  same. 

FIGURE  2.  —  Dorsal  view  of  a  normal  adult  having  two  plications  on 
the  fold.  Natural  size. 

FIGURE  2a.  —  Profile  of  the  same. 

FIGURE  2b.  —  Front  view  of  the  same.  (28th  Rept.  N.  Y.  State  Mus. 
Nat.  Hist.,  pi.  26,  figs.  23,  26,  25.) 

FIGURE  4.  —  The  cardinal  features  of  a  young  example  with  a  length 
of  3.25  mm. ;  showing  the  inception  of  the  deltidial  plates. 

FIGURE  4a.  —  Profile  of  the  same. 

FIGURE  5.  —  Cardinal  features  of  an  individual  which  has  attained  a 
length  of  6  mm.  The  umbo  has  become  incurved  and  the  development 
of  the  plates  is  well  advanced,  but  it  is  arrested  at  this  stage,  the  foramen 
not  becoming  enclosed  at  maturity. 

FIGURE  5a.  —  Outline  profile  of  the  same. 


CAMAROTCECHIA  NEGLECTA   HALL   (PAGE  341-344) 

FIGURE  6.  —  Dorsal  view  of  the  youngest  shell  observed ;  having  a 
length  of  .75  mm.,  and  a  width  of  .5  mm.  Though  the  foramen  is  open 
and  without  evidence  of  thickened  margins,  four  fine  plications  have 
already  appeared  on  the  dorsal  valve. 

FIGURE  6a.  —  Outline  profile  of  the  same. 

FIGURE  7.  — Dorsal  view  of  an  individual  with  a  length  of  2.25  mm. 
The  deltidial  plates  are  in  an  incipient  condition  and  the  plications  of 
the  surface  have  considerably  increased. 

FIGURE  7a.  —  Outline  profile  of  the  same. 

FIGURE  8.  —  Dorsal  view  of  a  mature  example.     Natural  size. 

FIGURE  8a.  —  The  same  in  profile.  (28th  Rept.  N.  Y.  State  Mus.  Nat. 
Hist.,  pi.  26,  figs.  3,  5.) 

FIGURE  3.  —  A  somewhat  abnormal  adult,  having  three  plications  on 
the  fold  and  the  lateral  plications  obsolescent.  X  2.  (llth  Ann.  Rept. 
Geol.  Surv.  Ind.,  pi.  27,  fig.  3.) 


CAMAROTCECHIA   ACINUS   HALL   (PAGES  339-341) 

FIGURE  9.  —  Ventral  view  of  a  young  shell  in  a  secondary  stage  of 
growth ;  showing  a  single  growth-line  and  the  embryonal  median  ridge. 

FIGURE  9a.  —  Dorsal  view  of  the  same.  The  foramen  is  slightly  nar- 
rowed, but  without  evidence  of  deltidial  plates;  the  embryonal  sinus  is 
well  defined. 

FIGURE  9b.  —  Outline  profile  of  the  same. 

FIGURE  10.  — Ventral  view  of  a  specimen  3.25  mm.  in  length. 

FIGURE  10a.  — Dorsal  view  of  the  same.  The  mature  fold  and  sinus 
have  not  yet  begun  to  develop  ;  the  foramen  shows  increased  constriction 
at  its  base,  and  slightly  thickened  margins. 

FIGURE  10ft.  —  Outline  profile  of  the  same. 

FIGURE  11.  —  Dorsal  view  of  an  average  adult,     x  4. 

FIGURE  lla.  — Profile  of  the  same. 

FIGURE  116.  —  Front  view  of  the  same;  showing  the  elevation  of  the 
median  fold. 

RHYNCHOTRETA   CUNEATA   DALMAN,   var.    AMERICANA 
HALL  (PAGES  351-356) 

See  Plate  XXII 

FIGURE  12.  —  Dorsal  view  of  a  shell  representing  the  earliest  stage  of 
growth  observed ;  length  1.5  mm.  The  foramen  is  widely  triangular  and 
unobstructed,  the  surface  is  covered  with  all  the  plications  of  maturity, 
and  the  sub-circular  valve  bears  a  broad  median  depression. 

FIGURE  12a.  —  Outline  profile  of  the  same. 

FIGURE  13.  —  Similar  view  of  a  shell  3  mm.  in  length.  The  outline 
has  become  elongate,  the  foramen  narrowed,  and  its  margins  thickened. 

FIGURE  13a.  —  Outline  profile  of  the  same. 

FIGURE  14.  —  Dorsal  view  of  a  normal  adult  individual;  showing  the 
elevated  beak  and  broad  plications.  Natural  size. 

FIGURE  14a.  —Profile  of  the  same;  showing  the  elevation  of  the  dor- 
sal fold. 

FIGURE  146.—  Front  view  of  the  same.  (28^  Rept.  N.  Y.  State  Mm. 
Nat.  Hist.,  pi.  25,  figs.  31,  34,  36.) 

FIGURE  15.  —  A  series  of  outlines  of  the  anterior  margin  ;  showing 
the  embryonal  sinus  or  depression  in  the  dorsal  valve  in  young  forms, 
reaching  its  maximum  in  individuals  having  a  length  of  4.5,  and  becom- 
ing obsolete,  or  the  line  of  junction  straight,  in  specimens  7  mm.  in 
length.  Outline  6  shows  the  inception  of  the  dorsal  fold.  In  8  the  four 
median  plications  are  distinctly  elevated  ;  9  is  from  a  normal  full-grown 
individual,  and  10  represents  the  maximum  elevation  of  the  fold.  The 
outlines,  except  in  9  and  10,  are  enlarged  to  the  diameter  of  a  fully 
developed  specimen. 

FIGURE  16.  —  Cardinal  area  of  figure  12. 

FIGURE  16«.  — Outline  profile  of  the  same. 


FIGURE  17.  —  Cardinal  area  of  figure  13. 

FIGURE  17a.  —  Outline  profile  of  the  same. 

FIGURE  18.  —  Cardinal  area  in  an  individual  4.5  mm.  in  length ;  show- 
ing the  considerably  advanced  development  of  the  deltidial  plates  and  the 
encroachment  of  the  foramen  on  the  apex  of  the  valve. 

FIGURE  18a.  —  Outline  profile  of  the  same. 

FIGURE  19.  —  Cardinal  area  at  a  length  of  5  mm. 

FIGURE  19a.  —  Outline  profile  of  the  same. 

FIGURE  20.  —  Cardinal  area  at  a  length  of  7  mm. 

FIGURE  20a.  —  Outline  profile  of  the  same. 

FIGURE  21.  — Cardinal  area  in  an  individual  which  has  attained  a 
length  of  12  mm.  The  plates  are  united  for  nearly  one-half  their  length, 
and  slightly  bent  outward  about  the  base  of  the  foramen. 

FIGURE  21a.  —  Outline  profile  of  the  same. 

FIGURE  22.  —  The  cardinal  area  at  maturity ;  showing  a  stronger 
flexion  of  the  plates  below  the  foramen,  and  a  sub-circular  foramen  en- 
croaching upon  the  apex  of  the  shell. 

FIGURE  22a.  —  Outline  profile  of  the  same. 


5a 


9a 


IGa 


17a 


18a 


17  .18 


fii?n\vTTT   TT^T   STT.TTRTAN   RuAOlHOPODA 


OF  THE 

UNIVERSITY 

OF 


PLATE   XIX 


PLATE   XIX 
HOIVKEOSPIRA  EVAX   HALL   (PAGES  360-365) 

See  Plate  XXII 

FIGURE  1.  —  Dorsal  view  of  the  youngest  individual  observed ;  having 
a  length  of  1  mm.  and  a  width  of  .8  mm.  The  ventral  umbo  is  erect, 
the  foramen  triangular  and  without  deltidial  plates ;  the  surface  bears 
two  faint  lines  of  growth,  and  outside  the  second  of  these  are  three  fine 
plications  on  each  side  of  a  median  sinus. 

FIGURE  la.  —  Outline  profile  of  the  same  ;  showing  the  very  shallow 
valves. 

FIGURE  2.  —  Dorsal  view  of  an  average  adult ;  showing  the  characters 
of  advanced  growth. 

FIGURE  2a.  —  Profile  of  the  same.  (28*7*  Rept.  N.  Y.  State  Mus.  Nat. 
Hist.,  pi  25,  figs.  14,  18.) 

FIGURE  3.  — Dorsal  view  of  an  immature  individual  5mm.  in  length, 
in  which  the  plications  on  the  earlier  portion  of  the  shell  end  abruptly  at 
a  growth-line,  from  there  outward  the  surface  characters  being  altogether 
primitive. 

FIGURE  3a.  —  Outline  profile  of  the  same. 

FIGURE  4.  —  The  cardinal  area  in  its  earliest  observed  condition ;  en- 
larged from  figure  1. 

FIGURE  5.  —  A  later  stage  of  growth  in  the  cardinal  area,  the  deltidial 
plates  having  a  considerable  development. 

FIGURE  6.  —  A  still  later  condition  of  this  area,  in  which  the  plates 
have  united,  enclosing  the  foramen. 

FIGURE  7. — The  cardinal  portions  of  an  individual  with  an  unusu- 
ally elevated  ventral  umbo ;  showing  also  an  advance  in  growth  from  the 
condition  represented  in  figure  6. 

FIGURE  8.  —  The  character  of  the  cardinal  area  in  a  normal  adult  of 
about  the  size  represented  in  figure  2.  The  foramen  has  become  circular, 
and  the  ventral  umbo  so  incurved  as  to  conceal  the  deltidial  plates. 

FIGURE  9.  —  A  small  obese  example  in  which  the  foramen  is  almost 
wholly  concealed. 

HOMCEOSPIRA  SOBRINA  SP.  NOV.  (PAGES  366-369) 

FIGURE  10.  —  The  youngest  shell  observed  ;  having  a  length  of  2  mm. 
and  a  width  of  1.6  mm.  The  shell  already  bears  two  plications  on  each 
side  of  the  median  sinus,  and  two  much  fainter  elevated  striae  in  the  sinus 
itself. 


FIGURE  10a.  —  Outline  profile  of  the  same. 

FIGURE  11.  —  Dorsal  view  of  an  adult  specimen,  somewhat  above  the 
average  size,  and  enlarged  to  11  diameters;  showing  the  normal  features 
of  maturity. 

FIGURE  lla.  —  Profile  of  the  same. 

FIGURE  116.  —  Ventral  view  of  the  same  individual.     Natural  size. 

FIGURE  12.  —  Transverse  section  of  an  individual ;  showing  the  spiral 
ribbon  and  the  number  of  volutions. 

FIGURE  13.  —  The  character  of  the  pedicle-passage  in  the  youngest 
example  (figure  10).  The  deltidial  plates  are  absent,  but  the  margins  of 
the  foramen  are  thickened. 

FIGURE  13«.  —  Outline  profile  of  the  same. 

FIGURE  14.  —  A  later  stage  of  growth,  in  which  the  plates  are  consid- 
erably developed. 

FIGURE  14a.  —  Outline  profile  of  the  same. 

FIGURE  15.  —  A  stage  of  growth  in  which  the  foramen  has  become 
circular  and  apical.  The  plates  are  slightly  flexed  along  two  oblique 
lines  which  converge  toward  the  base  of  the  area. 

FIGURE  15a.  —  Outline  profile  of  the  same. 

FIGURE  16.  —  The  mature  condition  of  the  pedicle-area,  the  deltidial 
plates  being  somewhat  concealed  by  the  incurvature  of  the  ventral 
umbo. 

FIGURE  16a.  —  Outline  profile  of  the  same. 

Figures  13-16a  are  drawn  to  a  scale. 


ATRYPINA   DISPARTLIS   HALL  (PAGES   369-372) 

FIGURE  17.  —  A  young  individual  2.5  mm.  in  length  ;  showing  surface 
and  pedicle  characters  essentially  as  at  maturity.  The  sinal  plications 
apparent  at  this  early  stage  appear,  in  the  usual  adult  form,  as  the  broad 
median  fold. 

FIGURE  17«. — Outline  profile  of  the  same;  showing  the  erect  beak 
and  shallow  valves. 

FIGURE  18.  —  A  young  individual ;  showing  asymmetry  in  the  devel- 
opment of  the  median  fold,  and  a  greater  number  of  lateral  plications 
than  in  the  normal  adult  (enlarged). 

FIGURE  19.  —  Dorsal  view  of  a  normal  adult. 

FIGURE  19«.  —  Ventral  view  of  the  same. 

FIGURE  19&.  —  Profile  of  the  same.  All  are  enlarged  to  two  diameters. 
(2Sth  Rept.  N.  Y.  Slate  Mus.  Nat.  Hist.,  pi.  25,  figs.  39-41.) 

FIGURES  20-23.  —  Enlarged  views  of  the  umbonal  region  of  the  ven- 
tral valve;  showing  the  variations  of  form  and  position  in  the  foramen 
as  observed  among  specimens  which  differ  but  little  in  actual  size. 


Plate  XIX 
3a 


f 


/•'S'"-"^S^fc^3w 


10  103 


lla 


-t 


T 


I3a 


15  15  a 


16a 


A 


4- 


22 


4 


4- 


STAGES  OF  GROWTH  IN  SILURIAN  BRACHIOPODA 


PLATE   XX 


PLATE   XX 


RETICULARIA  BICOSTATA,  VAR.  PETILA  HALL  (PAGES  380-382) 

FIGURES  1,  la.  — The  youngest  example  observed,  and  outline  profile 
of  the  same. 

FIGURE  2.  —  Dorsal  view  of  a  large  specimen. 

FIGURE  3.  — Cardinal  view  of  the  same.  X  3.  (\\thRept.  State  Geol. 
Ind.,  pi.  27,  figs.  8,  9.) 


SPIRIFER  CTRSPUS,  VAR.  SIMPLEX   HALL  (PAGES  380-382) 

FIGURE  4.- — Dorsal  view  of  the  youngest  growth-stage  observed. 
Within  the  single  growth-line,  the  smooth  sub-circular  initial  shell  is 
visible,  but  outside  this  the  shell  has  developed  the  median  fold  and  the 
surface  fimbriae. 

FIGURE  4a.  —  Outline  profile  of  the  same. 

FIGURE  5.  —  Dorsal  view  of  a  mature  individual.     X  2. 

FIGURE  5a.  —  Profile  of  the  same.  (2Sth  Ann.  Rept.  N.  Y.  State  Mus. 
Nat.  Hist.,  pi.  24,  figs.  1,  4.) 


SPIRIFER   CRISPUS   HISINGER   (PAGES  380-382) 

FIGURE  6.  —  Dorsal  view  of  a  young  form,  in  which  the  deltidial 
plates  are  in  an  incipient  condition. 

FIGURE  6a.  —  Outline  profile  of  the  same. 

FIGURE  7.  —  Dorsal  view  of  a  mature  specimen. 

FIGURE  la.  —  Profile  of  the  same.  (2Sth  Ann.  Rept.  N.  Y.  State  Mus. 
Nat.- Hist.,  pi.  24,  figs.  8,  12.) 


SPIRIFER  NIAGARENSIS   CONRAD   (PAGE  385) 

FIGURE  8.  —  The  cardinal  area  of  a  ventral  valve,  from  Lockport, 
New  York.  The  deltidial  plates  are  in  the  third  stage  of  development ; 
the  foramen  not  yet  enclosed,  but  confluent  with  the  hiatus  below  the 
basal  edges  of  the  plates.  Natural  size. 


SPIRIFER  RADIATUS   SOWERBY   (PAGES   382-386) 

FIGURE  9.  —  A  cardinal  view ;  showing  a  similar  stage  of  develop- 
ment of  the  deltidiuin  ;  from  an  individual  which  has  not  attained  normal 
full-growth. 

FIGURE  10.  —  The  youngest  growth-stage  observed. 

FIGURE  10a.  —  Outline  profile  of  the  same. 

FIGURE  11.  —  Dorsal  view  of  a  normal  adult. 

FIGURE  11  a.  —  Profile  of  the  same.  (28/7*  Ann.  Rept.  N.  Y.  State 
Mus.  Nat.  Hist.,  pi.  24,  figs.  23,  25.) 


ATRYPA   RETICULARIS   LINN^US   (PAGES   356-360) 

FIGURE  12.  —  The  incipient  shell ;  having  a  length  of  2.25  mm.  The 
deltidial  plates  have  already  begun  to  form,  and  the  shell  has  developed 
two  growth-lines. 

FIGURE  12«.  —  Outline  profile  of  the  same. 

FIGURE  13.  —  Ventral  view  of  a  full-grown  individual. 

FIGURE  13a.  —  Profile  of  the  same.  (28^  Ann.  Kept.  N.  Y.  State 
Mus.  Nat.  Hist.,  pi.  25,  figs.  46,  47.) 

FIGURE  14.  —  A  series  of  outlines  of  the  anterior  margin  of  the  valves; 
showing  the  rapid  increase  in  plications  and  the  reversion  from  the  em- 
bryonic fold  and  sinus  to  the  sinus  and  fold  of  maturity. 

FIGURE  15.  —  The  deltidial  characters  of  the  specimen  represented  in 
figure  12. 

FIGURE  15a.  — Outline  profile  of  the  same. 

FIGURE  16.  —  Similar  view  of  a  specimen  measuring  3X3  mm. 

FIGURE  16a.  —  Outline  profile  of  the  same. 

FIGURE  17.  —  Similar  view  of  a  specimen  measuring  3.5  X  4  mm. 

FIGURE  17a.  — Outline  profile  of  the  same. 

FIGURE  18.  —  Similar  view  of  a  specimen  measuring  8x7  mm. 

FIGURE  \Sa.  —  Outline  profile  of  the  same. 

FIGURE  19.  —  Similar  view  of  a  specimen  measuring  21  X  20  mm. 

FIGURE  19a.  — Outline  profile  of  the  same. 

FIGURE  20.  —  Similar  view  of  a  full-grown  specimen  measuring 
24  X  22  mm. 

FIGURE  20a. — Outline  profile  of  the  same. 


m:'  •   J  Ira 


4a 


5  a 


6a 


4 


15a 


19  a 


STAGES  OP  GROWTH  IN  SILURIAN  BUACHIOPODA 


20a 


PLATE   XXI 


PLATE   XXI 

MERISTINA   MARIA   HALL   (PAGES  377-380) 

FIGURE  1.  —  An  embryo  measuring  .75  X  .75  mm. ;  without  deltidial 
plates. 

FIGURE  la.  —  Outline  profile  of  the  same. 

FIGURE  2.  —  A  later  stage  of  growth,  at  which  the  shell  measures 
5x5  mm.,  and  the  deltidial  plates  have  developed  sufficiently  to  give 
the  foramen  a  circular  outline. 

FIGURE  2a.  — Outline  profile  of  the  same. 

FIGURE  3.  — Dorsal  view  of  a  normal,  mature  shell. 

FIGURE  3a.  —  Profile  of  the  same.  (28th  Ann.  Rept.  N.  Y.  State  Mus. 
Nat.  Hist.,  pi.  25,  figs.  9,  12.) 

MERISTINA   RECTIROSTRIS   HALL  (PAGES  372-374) 

FIGURE  4.  —  The  cardinal  features  at  an  extremely  early  stage  of 
growth,  the  shell  measuring  3x2  mm.  ;  showing  the  broad,  triangular 
foraminal  opening  and  its  faintly  thickened  margins. 

FIGURE  4a.  —  Outline  profile  of  the  same. 

FIGURE  5.  — The  same  features  when  the  shell  has  attained  a  size  of 
4.5  x  3.5  mm. ;  showing  the  gradual  approximation  of  the  sides  of  fora- 
men and  the  narrowing  of  the  umbo,  without  the  formation  of  deltidial 
plates. 

FIGURE  5a.  —  Outline  profile  of  the  same. 

FIGURE  13. —  The  same  features  at  maturity. 

FIGURE  I3a.  • —  Outline  profile  of  the  same. 

FIGURE  11.  —  Dorsal  view  of  a  young  individual. 

FIGURES  12,  I2a,  12ft.  —  Dorsal,  profile,  and  ventral  views  of  an  adult 
specimen,  (llth  Ann.  Rept.  GeoL  Surv.  Ind.,  pi.  27,  figs.  1,  10-12.) 

WHITFIELDELLA  NITIDA  HALL   (PAGES  374-377) 

FIGURE  10.  —  Dorsal  view  of  the  youngest  individual  observed;  hav- 
ing a  size  of  2.5  X  1.75  mm. 

FIGURE  !()«.  —  Outline  profile  of  the  same. 

FIGURE  6.  —  Cardinal  features  of  a  shell  having  dimensions  of  3  X  2 
mm. 

FIGURE  6a.  —  Outline  profile  of  the  same. 

FIGURE  7.  —  The  same  features  in  an  individual  having  a  size  of 
0x4  rnm.  In  this  example  the  umbo  is  abnormally  elongate,  giving  an 
unusual  prominence  to  the  deltidial  plates. 

FIGURE  7a.  —Outline  profile  of  the  same. 

FIGURE  8.  — The  same  features  at  a  size  of  5X4  mm.  ;  showing  the 
total  concealment,  from  incurvature,  of  the  deltidial  plates. 

FIGURE  8a.  —  Outline  profile  of  the  same. 

FIGURES  9,  9a,  9ft.  —  Dorsal,  ventral,  and  profile  views  of  the  normal 
adult.     (28th  Ann.  Rept.  N.  Y.  State  Mus.  Nat.  Hist.,  pi.  25,  figs.  3,  4,  5.) 


t 


6a 


8a 


9b 


Kb 


STAGES  OF  GROWTH  IN  SILURIAN  BRACIIIOPODA 


PLATE   XXII 


PLATE   XXII 

STAGES  OF  GROWTH  IN  SILURIAN  BRACHIOPODA 

(PAGE  312) 

Series  of  the  shells  of  Dalmanella  elegantula,  Orthothetes  subplanus,  Rhyn- 
chotreta  cuneata,  and  Homceospira  evax,  from  the  youngest  form  observed 
up  to  the  adult  or  senile  condition.  The  plate  was  drawn  on  stone 
from  a  photograph,  and  serves  to  show,  not  the  details  of  structure,  but 
the  character  and  completeness  of  the  material  which  has  served  as  the 
basis  of  this  work. 


V       OF  THE 

UNIVERSITY 


STAGES  OF  GROWTH  j 


Plate  XXII 


41   A  ^  A  A  A  A  4  4 


BRACHIOPODA 


JNIVERSITY 

OF 


I 


PLATE   XXIII 


PLATE   XXIII 
BILOBITES  ACUTILOBUS   RINGUEBERG  (PAGE  400) 

FIGURE  1 .  —  Outline  of  specimen  from  Niagara  Group,  Lockport, 
New  York.  X  4. 

BILOBITES   VERNEUILIANUS  LINDSTROM  (PAGE  400) 

FIGURE  2. —  Common  elongate  form  from  Upper  Silurian,  Gotland, 
Sweden.  X  4. 

BILOBITES   VARIOUS   CONRAD   (PAGES  402-405) 

FIGURE  3.  —  Dorsal  view  of  youngest  individual  observed ;  showing 
inception  of  radiating  striae  and  concealment  of  hinge-areas.  X  18. 

FIGURE  4.  —  Profile  of  same ;  showing  depth  and  extent  of  both 
valves.  X  18. 

FIGURE  5.  —  Hinge  view  of  preceding.     X  18. 

FIGURE  6.  —  Dorsal  side  of  specimen ;  showing  beginning  of  anterior 
marginal  sinus.  X  18. 

FIGURE  7.  —  Profile  of  same,     x  18. 

FIGURE  8.  —  Posterior  view  of  same.     X  18. 

FIGURE  9.  —  Dorsal  view  of  specimen,  figure  15  ;  showing  conceal- 
ment of  ventral  area.  X  9. 

FIGURE  10.  —  Ventral  view  of  same;  showing  dorsal  area.  X  9.  Com- 
pare this  with  dorsal  view  of  larger  specimen,  figure  21,  in  series. 

FIGURES  11-26.  —  Series  of  specimens;  seen  from  dorsal  side;  exhib- 
iting observed  stages  of  growth,  variation  and  development  of  hinge, 
hinge-area,  and  marginal  sinus.  X  4. 

FIGURE  27.  — Interior  of  ventral  valve;  showing  teeth,  muscular  im- 
pressions, minute  concave  plate  in  apex  of  fissure,  and  arrangement  of 
punctse  between  nodes  and  ribs,  x  6. 

Lower  Helderberg  Group,   Albany  county,  New  York. 

BILOBITES   BILOBUS   LINNI£   (PAGE  400) 

FIGURE  28.  —  Outline ;  showing  characteristic  form  of  this  species  as 
occurring  in  Upper  Silurian  of  Gotland,  Sweden. 


Plate  XXIII 


u 

o 


12 

O 


o 


DEVELOPMENT  or  BILOBITES 


PLATE  XXIV 


PLATE   XXIV 
BRACHIA  (PAGE  293) 

FIGURE  1.  — Brachial  supports  of  Macandrevia  cranium  Miiller;  nearly 
adult;  p,p,  points  of  former  attachment  to  a  septum  in  terebrataliform 
stage.  Enlarged. 

FIGURE  2.  —  Dorsal  valve  of  Terebratulina  septentrionalis  Couthouy, 
with  cirrated  brachia  attached ;  showing  relations  of  calcareous  loop, 
which  is  darkly  shaded  in  the  drawing. 

FIGURE  3.  —  Dorsal  valve  of  Magellania  kerguelenensis  Davidson,  with 
cirrated  brachia  attached  ;  showing  relations  of  calcareous  loop,  which  is 
darkly  shaded  in  the  drawing. 


TEREBRATALIA   OBSOLETA  DALL  (PAGES  406-408) 

FIGURE  4.  —  Loop  in  late  platidiform  stage,  with  cirrated  margin  of 
lophophore  attached.  X  15. 

FIGURE  5.  —  Side  view  of  lower  half  of  preceding ;  showing  cirrated 
edge  of  lophophore  passing  from  the  descending  branch  of  loop  to  sep- 
tum, and  over  ascending  branch.  X  15. 

FIGURE  6.  —  Dorsal  view  of  adult  shell.     Natural  size. 

FIGURE  7.  —  Exterior  of  ventral  valve.     Natural  size. 

FIGURE  8.  —  Profile  ;  showing  relative  convexity  of  valves.  Natural 
size. 

FIGURE  9.  —  Front  view.     Natural  size. 

FIGURE  10.  — Early  larval  brachiopod ;  interior  of  dorsal  valve ;  show- 
ing the  incomplete  circlet  of  tentacles  or  cirri,  visceral  mass,  and  some  of 
the  muscles  ;  ti,  li',  first  tentacles  developed ;  ti,  te',  second  pair  of  ten- 
tacles;  te,  tz',  third  pair  of  tentacles;  U,  U',  last  pair;  te,  new  tentacle 
just  growing  on  one  side  of  median  line;  m,  mouth  ;  ds,  dental  sockets; 
did,  diductor  muscles ;  ad,  adductor  muscles.  X  90. 

Figures  10,  11,  12,  are  drawn  from  specimens  which  had  been  dried 
and  afterward  expanded  by  soaking  in  water,  and  then  stained  and  pre- 
pared for  mounting.  The  original  proportions  and  relations  of  parts 
may  be  therefore  somewhat  altered. 

FIGURE  11.  —  Gwyniform  stage;  showing  complete  circle  of  cirri;  ps, 
pallial  sinus  ;  se,  setse  in  edge  of  shrunken  mantle.  X  60. 

FIGURE  12.  —  Cistelliform  stage ;  showing  deflection  of  growing  cir- 
rated edge  of  lophophore  by  dorsal  septum  (s) ;  m,  mouth ;  ad,  adductor 
muscles  ;  did,  diductor  muscles  ;  ds,  dental  sockets.  X  60. 


Plate  XXIV 


DEVELOPMENT  OF  TEREBRATALIA 


PLATE   XXV 


PLATE   XXV 

ONTOGENY  OF  THE  LOOP  IN  TEREBRATALIA  OBSOLETA 
DALL   (PAGES  408,  409) 

FIGURE  1.  —  Interior  of  dorsal  valve  in  early  cistelliform  stage ;  show- 
ing median  septum  and  dental  sockets.  X  25. 

FIGURE  2.  —  Side  view  of  septum  of  preceding.      X  25. 

FIGURE  3.  —  A  little  later  stage  ;  showing  growth  of  two  transverse 
expansions  on  septum.  X  25. 

FIGURE  4.  —  Side  view  of  same.     X  25. 

FIGURE  5.  —  Beginning  of  platidiform  stage  ;  showing  groove  on  edge 
of  septum,  with  arched  covering  at  posterior  end,  and  growth  of  crural 
processes.  X  25. 

FIGURE  6.  —  Side  view  of  same.     X  25. 

FIGURE  7.  —  Side  view  of  septum  at  a  later  stage,  just  before  growth 
of  descending  branches  of  loop ;  showing  form  and  position  of  ascending- 
branches,  or  secondary  loop,  and  septal  characters.  X  25. 

FIGURE  8.  —  Platidiform  stage ;  showing  union  of  descending  branches 
with  septum.  X  12. 

FIGURE  9.  —  Beginning  of  ismeniform  stage;  showing  growth  of  as- 
cending branches.  X  12. 

FIGURE  10.  —  Early  muhlfeldtiform  stage ;  showing  holes  developed  in 
ascending  branches  by  resorption.  X  12. 

FIGURE  11. — Oblique  view  of  septum,  with  ascending  branches 
broken  off.  X  12. 

FIGURE  12.  —  Beginning  of  terebrataliform  stage ;  showing  union  of 
ascending  with  descending  branches.  X  12. 

FIGURE  13.  —  A  little  later  stage ;  showing  resorption  of  broad  de- 
scending branches.  X  7. 

FIGURE  14.  —  Late  adolescent  stage  ;  showing  connecting  bands  from 
descending  lamellae  to  septum.  X  2|. 

FIGURE  15.  — Fully  adult  loop,  with  connecting  bands.     X  2^. 


DEVELOPMENT  PF  TEBEBRATALIA 


OF  THE 

IVERSITY 


PLATE   XXVI 


PLATE   XXVI 
DIELASMA   TURGIDUM  HALL  (PAGES  412,  413) 

FIGURE  1.  —  The  centronelliform  stage  of  the  loop;  ventral  view. 
X9. 

FIGURE  2.  —  A  later  stage ;  showing  the  resorption  of  the  anterior  por- 
tion of  the  loop.  X  6. 

FIGURE  3.  —  Early  Dielasma  stage,  produced  by  further  resorption  of 
the  centronelloid  loop.  X  6. 

FIGURE  4.  —  Loop  and  crural  plates  of  mature  specimen,     x  6. 

FIGURE  5.  — Profile  of  the  connecting  band,     x  6. 

FIGURE  6.  —  Side  view  of  the  loop,  crura,  and  septum,    x  6. 

St.  Louis  Group,  Kentucky. 

ZYGOSPTRA  RECURVIROSTRIS  HALL  (PAGES  413-416) 

FIGURE  7.  —  Centronelliform  stage  of  the  loop.     X  12. 

FIGURE  8.  —  Profile  of  same.     X  12. 

FIGURE  9.  —  A  later  stage ;  showing  partial  resorption  of  loop  in 
front  and  greater  divergence  of  descending  branches.  X  12. 

FIGURE  10.  —  The  same  ;  profile.     X  12. 

FIGURE  11.  —  A  little  more  advanced  stage ;  showing  increased  length 
of  connecting  band.  X  12. 

Specimens  figures  7-11  are  from  the  Trenton  Shales,  St.  Paul, 
Minnesota. 

FIGURE  12.  —  A  looped  stage,  with  broad,  curved,  descending  branches 
and  more  slender  transverse  band.  X  6. 

FIGURE  13.  —  The  same;  profile.     X  6. 

FIGURE  14.  —  A  later  stage;  showing  more  slender  loop.     X  6. 

FIGURE  15.  —  A  specimen  showing  curved,  diverging,  descending 
branches,  long  trans  verse  band,  and  two  projections  of  the  lamellae,  which 
are  the  beginning  of  the  spiral  cones.  X  6. 

FIGURE  16.  —  A  subsequent  stage  in  which  the  lamellae  are  more  ex- 
tended and  ventrally  and  inwardly  curved.  X  6. 

FIGURE  17.  —  The  same ;  profile.     X  6. 

FIGURE  18. — A  young  individual  in  which  there  are  about  one  and 
one-half  turns  to  each  spiral.  X  6. 

FIGURE  19.  —  The  same ;  profile,     x  6. 

FIGURE  20.  — The  brachial  supports  in  a  mature  specimen.     X  6. 

The  specimens  figures  12-20  are  from  the  top  of  the  Trenton,  Frank- 
fort, Kentucky. 

FIGURE  21.  — The  spirals  and  loop  in  a  Canadian  specimen  of  (Ana- 
zyga  =)  Zygospira  recurvirostris  Hall.  (After  Davidson.)  X  3. 

FIGURE  22.  —  The  spirals  and  loop  in  (Hallina  =}  Zygospira  Saffordi 
Winchell  and  Schuchert.  Trenton,  Tennessee.  X  6. 

FIGURE  23.  —  The  spirals  and  loop  in  (Hallina  =)  Zygospira  Nicolletti 
Winchell  and  Schuchert.  Trenton,  Minnesota,  x  6. 

FIGURE  24.  —  The  spirals  and  loop  in  Catazyga  Headi  Billings. 
(After  Hall.)  X2. 

FIGURE  25.  —  The  spirals  and  loop  in  Zygospira  modesta  Say.  (After 
Hall.)  x  2£. 

FIGURE  26.  —  The  same.     (After  Davidson.)     X  3£. 


BRACHIAL  SUPPORTS  IN  DIELASMA  AND  ZYGOSPIRA 


PLATE   XXVII 


PLATE   XXVII 
PLEURODICTYUM  LENTICULARE  HALL  (PAGES  422-425) 

FIGURE  1 .  —  Lower  side  of  initial  cell,  or  nepionic  stage,     x  3^. 

FIGURE  2.  —  Lower  side  of  initial  cell  with  one  bud  ;  first  neanic 
stage.  X  3£. 

FIGURE  3.  —  Initial  cell  with  two  buds  ;  second  neanic  stage.     X  3£. 

FIGURE  4.  — Initial  cell  with  three  buds  ;  third  neanic  stage.     X  3£. 

FIGURE  5.  —  Initial  cell  with  four  buds;  fourth  neanic  stage.     X  3£. 

FIGURE  6.  —  Completed  neanic  stage;  showing  initial  corallite  (1) 
and  seven  peripheral  corallites  (2-8).  X  3^. 

FIGURE  7.  —  Interior  of  nepionic  corallite.     X  3J. 

FIGURE  8.  —  Exterior  of  same  specimen.     X  3^. 

FIGURE  9.  —  Upper  side  of  specimen  representing  first  neanic  or 
Aulopora  stage ;  consisting  of  nepionic  cell  and  one  bud.  Apex  of  bud 
opens  into  visceral  cavity  of  parent  cell.  X  3£. 

FIGURE  10.  —  Profile  of  same;  showing  oblique  apertures  of  corallites, 
and  thickened  margin  of  parent  cell.  X  3^. 

FIGURE  11.  — Lower  side  of  preceding.     X  3£. 

FIGURE  12.  —  Upper  side  of  completed  neanic  stage;  showing  incep- 
tion of  eighth  cell.  X  3£. 

Figures  1-6  are  taken  from  epithecal  lines  of  growth  shown  in  Plate 
XXVIII,  figure  2.  Remaining  figures  are  from  actual  specimens. 
Numbers  refer  to  order  of  calical  succession. 

Lower  Helderberg  Group,  Albany  county,  New  York. 


DEVELOPMENT  or  PLEURODICTYUM 


PLATE  XXVIII 


PLATE   XXVIII 
PLEURODICTYUM  LENTICULARE  HALL   (PAGES  423,  424) 

FIGURE  1.  — Lower  or  epithecal  side  of  specimen  ;  showing  successive 
alternate  gemmation  from  parent  corallite.  X  3£. 

FIGURE  2.  —  Similar  specimen  with  lines  of  growth  more  strongly 
marked.  The  order  of  budding  is  opposite  to  that  of  preceding  speci- 
men. X  3£. 

Lower  Helderberg  Group,  Albany  county,  New  York. 


Plate  XXVIII 


PLEURODICTYUM 


PLATE  XXIX 


PLATE   XXIX 
PLEURODICTYUM  LENTICULARE   HALL   (PAGES  423,424) 

FIGURE  1.  — Outline  of  calices  of  specimen  Plate  XXVIII,  figure  2; 
showing  central  primary  cell  and  seven  peripheral  calices  numbered  in 
the  order  of  their  development.  X  3^. 

FIGURE  2.  —  The  same  ;  side  view. 

Lower  Helderberg  Group,  Albany  county,  New  York. 


Plate  XXIX 


PLEURODICTYUM 


PLATE   XXX 


PLATE   XXX 
PLEURODICTYUM  LENTICULARE  HALL   (PAGES  423,  424) 

FIGURE  1.  — Outline  of  upper  side  of  specimen  with  eight  peripheral 
calices.  X  3£. 

FIGURE  2. — Upper  side  of  symmetrical  specimen;  showing  general 
features  of  calices,  mural  pores  in  central  corallite,  and  thickened  epithe- 
cal  border.  X  3£. 

Lower  Helderberg  Group,  Albany  county,  New  York. 


Plate  XXX 


PLEURODICTTUM 


PLATE  XXXI 


PLATE   XXXI 


PLEURODICTYUM  LENTICULARE   HALL   (PAGE  424) 

FIGURE  1.  —  Calical  diagram  of  gerontic  specimen;  showing  en- 
largement of  second,  fourth,  and  eighth  corallites,  and  the  addition  of 
tertiary  cells,  forming  a  second  series  of  peripheral  calices.  X  3^. 
Lower  Helderberg  Group,  Albany  county,  New  York. 


PLEURODICTYUM   PROBLEMATICUM   GOLDFUSS   (PAGE  426) 

FIGURE  2.  — Lower  side  of  cast  of  corallum  with  epitheca  removed  ; 
showing  proximal  extremities  of  several  corallites.  Upper  edge  of  figure 
represents  portion  of  periphery  of  corallum.  Thus,  lower  angle  of  each 
corallite  represents  the  point  of  budding  from  parent  cell,  and  is  con- 
nected with  it  by  a  pore,  shown  for  three  of  the  corallites  by  dotted  lines 
from  p.  It  will  be  noticed  that  all  the  pores  in  the  angles  are  larger  than 
the  others.  Otherwise  these  and  the  initial  pores  cannot  be  distin- 
guished from  the  ordinary  mural  pores  between  the  flat  sides  of  the 
corallites.  X  7.  Devonian,  Pelm,  Germany. 


FAVOSITES  EPIDERMATUS?   ROMINGER   (PAGE  426) 

FIGURE  3.  —  Side  view  of  mature  corallite  with  attached  intermural 
bud.  Specimen  broken  from  interior  of  a  large  colony.  X  3^. 

FIGURE  4.  —  The  same,  front  view,  with  bud  removed  ;  showing  pore 
or  mural  opening  (p)  at  lower  point  of  attachment  of  bud,  corresponding 
to  those  indicated  in  figure  2.  X  3£. 

Corniferous  Limestone,  Cherry  Valley,  New  York. 


Plate  XXXI 


PLEDRODICTYUM  AND  FAVOSITES 


PLATE    XXXII 


PLATE   XXXII 
MICHEL1NIA   CONVEXA  D'ORBIGNY   (PAGES  431,  432) 

FIGURE  1.  —  Diagrammatic  representation  of  upper  surface  of  coral- 
lum;  consisting  of  parent  cell  (A)  and  six  peripheral  corallites  (Jf,  #, 
3,  etc.). 

FIGURE  2.  — The  same;  showing  the  introduction  of  three  triangular 
intermural  buds  (./',  2',  3',  etc.). 

FIGURE  3.  —  Third  condition  ;  with  six  triangular  buds  about  the 
parent  corallite,  and  three  on  the  periphery  of  the  corallum. 

FIGURE  4.  —  Top  of  corallum ;  showing  further  growth  of  preceding 
corallites,  with  the  addition  of  three  peripheral  calices  (  4",  5",  6"). 

FIGURE  5.  —  The  same  during  a  succeeding  stage  ;  showing  increase 
in  size  of  corallites,  and  modifications  produced. 

FIGURE  6.  —  Completed  growth  of  first  system  of  intermural  gemma- 
tion ;  showing  dissociation  of  original  series  of  corallites  (A,  1, 2,  3,  etc.), 
and  representing  condition  preparatory  for  new  series  of  interstitial 
corallites. 

All  the  figures  are  natural  size. 


Plate  XXXII 


FAVOSITID^E 


PLATE   XXXIII 


PLATE   XXXIII 
MICHELINIA   CON  VEX  A   D'ORBIGNY   (PAGE  432) 

FIGURE  1.  — Development  of  a  group  of  corallites  from  initial  conical 
cell  to  corallum  with  nineteen  calices.  The  figure  represents  parallel 
horizontal  sections  through  the  corallum  ;  showing  the  number  and  form 
of  the  calices,  their  order  of  development,  and  the  modifications  taking 
place  during  growth.  The  parent  cell  is  marked  A ;  first  series  of  calices, 
1,  2,  8,  etc.  ;  first  series  of  intermural  buds,  1',  2',  3',  etc. ;  peripheral 
series,  1",  2",  3",  etc.  Notation  corresponds  with  that  of  preceding  plate. 
Natural  size. 


Plate  XXXIII 


FAVOSITID^J 


PLATE  XXXIV 


PLATE   XXXIV 
TORNOCERAS  (PAGES  437-439) 

FIGURE  1.  —  Protoconch ;  showing  first  septum  with  lateral  edges 
broken.  X  18. 

FIGURE  2.  —  Side  view  of  preceding.     X  18. 

FIGURE  3.  —  Ventral  view  of  protoconch  with  one  attached  air-cham- 
ber; showing  siphonal  caecum.  X  18. 

FIGURE  4.  —  Side  view  of  first  whorl.     X  18. 

FIGURE  5.  —  Ventral  view  of  preceding ;  showing  development  of 
ventral  lobe.  X  18. 

FIGURE  6.  — •  Transverse  section  of  two  whorls  near  the  proto- 
conch. X  18. 

FIGURE  7.  — Ventral  side  of  protoconch,  retaining  the  shell.     X  18. 

FIGURE  8.  —  Yentral  side  of  specimen  with  first  chamber;  showing  sur- 
face ornaments  and  indication  of  siphonal  caecum.  X  18. 

FIGURE  9.  —  Profile  of  same.     X  18. 

FIGURE  10.  — Surface  ornaments  on  a  specimen  consisting  of  a  single 
whorl.  X  18. 

FIGURE  11.  —  Four  striae  from  the  second  whorl  of  a  specimen ;  show- 
ing funnel  sinus.  X  18. 

FIGURE  12.  —  Vertical  section  ;  showing  septa  and  air-chambers.  X  18. 

FIGURE  13.  —  Outline  of  a  half -grown  specimen,     x  3. 

FIGURE  14.  —  A  series  of  developed  septa  beginning  with  the  first ; 
showing  gradual  inception  and  formation  of  lobes  and  saddles  to  the 
adult  period  represented  by/,  a,  b,  c,  d,  represent  the  1st,  2d,  3d,  and 
4th  septa,  while  e  and /represent  the  7th  and  8th,  respectively.  X  9. 

Specimens  in  Yale  University  Museum,  from  the  Hamilton  Shales  of 
Wende  Station,  New  York,  except  specimen  figure  13,  which  is  from 
18-Mile  Creek,  New  York. 


TORNOCERAS 


INDEX 


INDEX 


Acantharia,  composite  spine  in,  49. 
Acanthaster  Solaris,  protective  spines  in, 

56. 

acanthogeny,  19. 
Acanthosicyos    horrida,   cultivation    of, 

Henslow,  74. 
Acanthothyris,  245. 

Doderleini,  spines  in,  51. 
spinosa,  spines  in,  51. 
Acaste,  129,  156. 

acceleration  in  Brachiopoda,  modifica- 
tions from,  233. 
Acerocare,  142,  145. 
Achatinella,  differentiation  of,  65. 

variations  in,  36,  37. 
Achatinellae,  free  variation  in,  Hyatt 
and  Verrill,  36. 
specific    differentiation    in,  Hyatt 

and  Verrill,  36. 
Achtheres,  variation  in  anterior  antennae 

of,  192. 

Acidaspidaj,  129,  131,  139,  157. 
defined,  151. 

extremes  of  spinosity  in,  56. 
lobes  in  glabella  of,  149. 
Acidaspis,  70,  113,  129,  149,  151,  152, 
157,  174,  176,  179,  210. 
accelerated  development  of  spines 

in,  22. 
eyes  in,  126. 

features  of  dorsal  shield  of,  182. 
glabella  in  larval,  126,  180. 
high  specialization  of,  174. 
no  eye-line  in,  180. 
spiniform  processes  in,  78. 
third  segment  in,  127. 
tuberculata,  173. 
Acontaspis    hastata,    multiplication    of 

spines  in,  69. 
Acontheus,  cephalon  in,  182. 


Acrocephalites,  142. 

Acrosoma  arcuata,  protective  mimetic 

features  in,  62. 
Acrothele,  244. 

marginal  upper  beak  in,  236. 

pedicle-opening  in,  241. 
Acrotreta,  244. 

marginal  upper  beak  in,  236. 
Actinocrinus  lobatus,  spines  in,  51. 

pernodosus,  spines  in,  51. 
Actinopeltis,  155,  156. 
Actinopteria  Boydi,  spines  in,  51. 
Adouin's  law  regarding  the  Articulata, 

83. 
^Eglina,  113,  132,  146,  188. 

armata,  eyes  in,  147. 

free-cheeks  in,  117,  118. 

eyes  in,  147. 

free-cheeks  in,  124. 

princeps,  eyes  in,  147. 

rediviva,  eyes  in,  147. 
Agassiz,  A.,  homologous  structures  in 
Echinodermata,  94. 

L.,  foundation  of  orders,  242. 
interpretation  of  family  charac- 
ters, 290. 

ontogeny  a  standard  of  classifica- 
tion, 120. 
Aglaspidse,  132. 
Aglaspis,  132. 
Agnostidas,  130,  131,  134. 

defined,  135. 

primitive  head  structure  in,  135. 

primitive  structure  in,  135. 
Agnostini,  134. 
Agnostus,  130,  136,  176,  178. 

absence  of  eyes  in,  183. 

Cambrian,  183. 

cephalon  in,  127,  128. 

distinct  plate  in,  134. 

eye-spots  lost  in,  135. 

free-cheeks  in,  124. 


600 


INDEX 


free  segments  in,  136. 

free  thoracic  segments  in,  1 83. 

generic  names  for,  Barrande,Corda, 

136. 

hypostoma  in,  135. 
nudus,  177. 

youngest  forms  of,  176. 
pleural  spines  in,  80. 
pygidium  in,  135. 
rex,  177. 

youngest  forms  of,  176. 
series  of,  in  Museum  of  Compara- 
tive Zoology,  134. 

in  National  Museum,  134. 
Agraulus,  142,  145. 
Agulhasia,  291,  307. 

hinge-plates  and  muscular  fulcra  in, 

305. 

Allorchestes  armatus  in  Lake  Titicaca, 
Faxon,  65. 
evolution    of,    in    Lake    Titicaca, 

Faxon,  36. 
variations  in,  37. 
wonderful  specific  development  of, 

in  Lake  Titicaca,  Faxon,  65. 
Alveopora,  428. 
Amboccelia,  245. 

ammonites,     retrogression     in    Creta- 
ceous, 91. 

Ammonoidea,  chief  spiny  forms  of,  96. 
geological  development  of,  Packard, 

98. 

radicle  types  in,  95. 
Amoeba  proteus,  form  of,  27,  28. 
Amcebina,  43. 
Amphiclina,  245,  270. 
Amphigenia,  245. 

dorsal  beak  in,  264. 
Amphion,  155,  156. 
amphipods  found  in  lakes  Baikal  and 

Titicaca,  64. 

Amphiuma,  external  limbs  in,  76. 
Ampycini,  134. 

Ampyx,  113,  130,  138,  220,  221. 
eye-spots  lost  in,  135. 
fourth  annulation  in,  127. 
free-cheeks  in,  135. 
structure  of  sutures  in,  Angelin, 

134. 
Anacheirurus,  155. 

pentamerous  lobation  of  glabella 
in,  155. 


anagerontic  stage,  248. 
Anastrophia,  387. 

absence  of  embryonal  sinus  in,  388. 
internascens,  315. 

discussion  of,  337. 
valves  in,  388. 
Ancistocrania,  244. 
Ancylobrachia,  290,  411,  412,  417. 
centronelliform  loop  in,  411. 
classification  of,  305. 
loop  in,  410. 
Ancyloceras,  96. 
Ancyropyge,  151,  152. 
Aneucanthus,  140. 
animals,  defence  in,  Morris,  52. 

and  plants,  limitations  of  the  forms 

of  species  of,  7. 

parasitic,  reduction  in  limbs  of,  83. 
Annelida,  252. 
annelids,  mouth  of,  255. 

Waldron,  313. 
Angelin,  N.  P.,  structure  of  sutures  in 

Ampyx,  134. 
Angelina,  142,  145. 
Anomia,  399. 

aculeata,  prodissoconch  of  young, 
20. 

spines  in,  51. 
byssal  plug  of,  38. 
discoid  form  of  shell  in,  287. 
phylogeny  of,  23. 
smooth  prodissoconch  of,  18. 
Anomocare,  142. 
Anopolenus,  142. 

antelope  of  America,  prong-horn,  com- 
pound antlers  in,  68. 
antenn£e,  anterior,  in  Triarthrus,  205. 

posterior,  in  Triarthrus,  206. 
antennules  in  Triarthrus,  205. 
Anthrodesmus  octocornis,   spine  growth 

from  frustule  of,  43. 
antler,  definition  of,  9. 
antlers,  compound,  in  deer  family,  68. 
in    prong-horn     antelope    of 

America,  68. 

in  both  sexes  of  Procervulus,  57. 
in  deer  and  elk,  9. 
of  deer,  21. 
of  fallow  deer,  53. 
simple,  in  Tertiary  deer,  52. 

of  young  deer  and  elk,  Lydek- 
ker,  24. 


INDEX 


601 


Anurcea  squamula,  spines  on  loricse  of 

45. 

anus  in  Triarthrus,  209. 
Aphaneropegmata,  242. 
Apteryx  australis,  vestiges  of  wings  in, 

85. 

Apus,  185,  186,  187,  188,  203,  209,  210, 
211,  215. 

development  of,  Balfour,  189. 

nauplian  features  of  adult,  189. 

nauplius  of,  164,  191. 

studies  of,  Bernard,  118. 

under  side  of  head  of,  210. 
arachnids,     trilobites     and     limuloids 

classed  with,  109. 
archenteron  in  Cistella,  248. 
Archiaspides,  Haeckel,  112,  113. 
Arctinurus,  150. 

glabella  in,  150. 
Areia,  153. 

cephalon  in,  154. 
Arethusina,  113,  148,  178. 

archaic  eye-lines  in,  148. 

eye-line  in,  223. 

fixed-cheeks  in,  148. 

Konincki,  youngest  specimens  of, 

177. 
Arges,  150,  151,  174. 

consanguineus,  Clarke,  174. 

extremes  of  spinosity  in,  56. 

eyes  in,  126. 

glabella  in,  1 50. 
Argiope,  238. 

Arraphus  based  on  Harpes,  137. 
Artemia,  first  pair  of  appendages  of, 

191. 

Arthropoda,  accelerated  development  of 
spines  in,  22. 

extinct,  183. 

geological  range  and  distribution 

of,  184. 

Arthropomata,  Owen,  242. 
Articulata,  absence  of  anal  opening  in, 
264. 

Adouin's  law  regarding,  83. 

Huxley,  242. 

spine  production  in,  13. 
articulates,  classification  of,   Haeckel, 

112. 

Asaphdindi  142,  145. 
Asaphelhts,  146. 
Asaphida,  Haeckel,  112. 


Asaphidge,  129,  131,  139,  146. 

defined,  145. 
Asaphiui,  134, 139,  152. 
Asaphiscus,  146. 

Asaphus,  116,  129,  145,  146,  222. 
a  normal  trilobite  type,  128. 
free  cephalic  segments  in,  118. 
glabella  in,  135. 
gnathobases  of,  207. 
pleural  spines  in,  80. 
Aspergillum,  237. 

Aspidonia,  Merostomata    included    in, 
112. 

Trilobita  classed  with,  112. 
Astacus,  embryos  of,  192. 
Asteroidea,  compound  spines  in,  68. 
development  of  tubercles  of,  into 
spines,  Wachsmuth  and  Springer, 
50. 

smooth  surface  in  early,  25. 
spine  growth  in,  69. 
Astragalus      Tragacantha,     leaves     of, 
transformed    into   spines,    75. 

spiniform  leaf  axis  of,  73,  75. 
Astropilium  elegans,  spines  in,  48. 
Athyridas,  283. 

brachial  supports  in,  276. 
diverging  cones  in,  283. 
fleshy  portions  of  brachia  in,  283. 
primary  lamellae  in,  416. 
Athyris,  245,  284,  387. 

dorsal  beak  in,  264. 
Atops,  129,  139,  140,  141,153. 
cephalon  of,  1 28. 
characters  of,  141. 
glabella  in,  141. 
trilineatus,  141. 

characters  of,  141. 
Atremata,  242,  244,  245,  247,  267,  282. 

defined,  243. 
Atretia,  231,  245,  271. 

protegulum  in,  234. 
Atrypa,  245,  284,  395,  416. 
aspera,  24,  25. 
disunited  loop  in,  292. 
dorsal  beak  in,  264. 
hystrix,  24,  25. 

marginal  spines  of,  Hall  and 

Clarke,  45. 

reticularis,  24,   25,   315,   327,   350, 
352. 
beak  in,  389. 


602 


INDEX 


circular  apical  perforation  in, 

389. 

discussion  of,  356. 
sinus  and  fold  in,  388. 
variation  in,  66. 
variations    in    stock  of,  Wil- 
liams, 25. 
volutions    in    spiral    cone    of 

mature,  417. 
rugosa,  357. 
spinosa,  25. 

spines  in,  51. 
tumida,  377. 

whorls  in  spirals  of,  416. 
young  condition  in,  417. 
Atrypidse,  283,  416. 

brachial  supports  in,  276. 
converging  cones  in,  283. 
origin  of,  417. 
whorls  in  spirals  of,  416. 
Atrypina  disparilis,  315,  364. 

discussion  of,  369. 

Attheya    decora,    spine    growth    from 
frustule  of,  43. 

spines  from  angles  of,  44. 
Aulocrinus  Agassizi,  spines  on  terminal 

sacs  of,  45. 
Aulopora,  425,  427. 
corallites  in,  427. 
subtenuis,  427. 
visceral  cavities  in,  425. 
Aulosteges,  69,  70,  96,  245,  270. 

spines  on  deltidium  of,  260. 
Avalonia,  140. 

glabella  in,  141. 
Avicula,  255. 

dissoconch  growth  in,  23. 
smooth  prodissoconch  of,  1 8. 
sterna,  nepionic  stages  of,  20. 
prodissoconch  of,  20. 
spines  in,  51. 

Aviculidse  and  allied  forms,  Lower  Si- 
lurian nuculoid  radicle  for,  23. 
Aviculopecten,  23. 

ornatus,  spines  in,  51. 
scabridus,  spines  in,  51. 


Bactrynium,  245. 

Baculites  a  typical  phylogerontic  form, 
Hyatt,  91. 


Bailey,  L.  H.,  culmination  and  descent 
of  Echinodermata,  26. 
spinous  processes  on  the  plant  body, 

90. 

Bailiella,  140. 
Baileyi,  178. 
glabella  in,  141. 

Balanophoreae,  spiniform  scales  in,  83. 
Balanus,  188. 

tintinnabulum,  var.  spinosus,  spines 

on,  52. 

Balfour,  F.  M.,  defensive  spines  in  nau- 
plius  larva  of  Lepas  fascicularis, 
56. 

development  of  Apus,  189. 
variation  in  nauplius  of  trilobites, 

191. 

barberry,  disappearance  of  spines  on, 
Lothelier,  74. 

indirect  growth  of  spines  in,  12, 13, 

14. 
leaves  of,  transformed  into  spines, 

75. 

thorns  of,  10. 
barnacles,  spines  on  shells  of,  Darwin, 

52. 

Barrande,  J.,  classification  of  trilobites, 
110,  132. 
description  of  elementary  forms  of 

trilobites,  178. 
determination  of  sutures  in  Trinu- 

cleus,  Dionide,  and  Harpes,  134. 
development  of  trilobites,  310. 
generic  names  for  Agnostus,  136. 
metamorphoses  of  trilobites,  166. 
orders  of  development  in  trilobites, 

167. 

possible  trilobite  eggs,  169. 
Sao  hirsuta,  166. 
Barrandia,  146. 
Basilicus,  146. 

Bateson,  W.,  horns  in  some  Hercules 
beetles,  59. 
repetition  of  parts  universal  among 

organisms,  67. 
bathmic  energy,  34. 
force,  Cope,  29. 
Bathynotus,  140. 

glabella  in,  141. 
Bathyurellus,  146. 
Bathyuriscus,  146. 
Bathyurus,  146. 


INDEX 


603 


batrachians,  horned,  position  of,  in  de 
fence,  54. 

snake-like,  external  limbs  of,  76. 
Bavarilla,  142. 
beetle,  stag,  horns  in,  60. 
beetles,  Hercules,  horns  in  some,  Bate- 
son,  59. 

Bellinurus,  222. 
Berberis     vulgaris,     disappearance    of 
spines  on,  74. 

indirect  growth  of  spines  in,  12. 
Bergeronia,  142. 
Bernard,  H.  M.,  affinities  of  trilobites, 

110. 

cranidium  in  trilobites,  181. 
development  of   crustacean   head, 

123. 

dorsal  shield  of  trilobites,  193. 
hypostoma  in  trilobites,  188. 
phyllopod   affinities    of    Trilobita, 

210. 

Btfida,  245. 
Bignonia  argyroviolacea,  leaves  in,  Ker- 

ner,  89,  90. 
Billings,   E.,   diagnosis   of    the  genus 

Eichwaldia,  336. 
Bilobites,  245,  399,  405. 
acutilobus,  404. 
bilobus,  315,  400,  404. 

early  neanic  condition  of,  270. 
var.  Verneuilianus,  400,  401. 
development  of,  399. 
gerontic  development  of,  270. 
various,  400,  401. 

developmental  changes  in,  402. 
nepionic  shell  in,  404. 
obsolescence  of  bilobed  form 

of  shell  in,  270. 
ontogeny  of,  404. 
Verneuilianus,  404. 

early  neanic  condition  of,  270. 
bird,  wingless,  of  New  Zealand,  vestiges 

of  wings  in,  85. 
birds,  spur  on  legs  and  wings  of,  9. 

spurs  on,  59. 

blackberry,  prickles  on,  9. 
blastosphere,  248. 
Boeclcia,  142,  145. 
Bohemilla,  132. 
Bohemillidae,  132. 
Bolbocephalus,  146. 
Bolivina  robusta,  single  spine  in,  44. 


Bouchardia,  245,  288,  294,  295, 301,  303, 
308. 
hinge-plates  and  muscular  fulcra  in, 

305. 

plectolophus  stage  in,  281. 
bouchardiform  stage,  292. 
box-fish,  spiny,  70. 

protective  spines  in,  55. 
brachia,  morphology  of,  274. 
brachial  structures,  classification  of,  276. 
brachidium  in  Terebratulacea  and  Spi- 
riferacea,  274. 

of  brachiopods,  274. 
Brachiopoda,  230,  247,  327. 
affinities  of,  232. 
alternating  cirri  in    adult    lopho- 

phore  of,  Hancock,  275. 
application  of  law  of  radial  sym- 
metry to,  236. 

beginning  of  lophophore  in,  277. 
brachidium  of,  274. 
classification  of,  305. 

Dall,    Deslongchamps,    Gray, 

Waagen,  290. 
of  stages  of  growth  and  decline 

of,  246. 

comparisons  and  homologies  in,  253. 
compound  spines  in,  68. 
correlations  of  ontogeny  and  phy- 

logeny  in,  286. 

deltidium  in,  Deslongchamps,  391. 
development  of,  229. 
development  of  some  Silurian,  310. 
spines  in  shells  of,  13,  14. 
spinose    forms    of,  Hall   and 

Clarke,  24,  25. 
difference  in  valves  in,  234. 
dorsal  foramen  in  some,  King,  Hall, 

GEhlert,  264. 
embryonic  stages  of,  247. 

Waldron,  313. 
ephebic  period  in,  269. 
evolution  of,  30,  31. 
families  of,  CEhlert,  243. 
genesis  of  form  in,  238. 
genetic  sequence  in  forms  of,  24. 
geological  development  of,  Pack- 
ard, 98. 
gerontic  period  in,  Clarke,  269. 

stages  in,  398. 
largest  species  of,  96. 
larval  stages  of,  250. 


604 


INDEX 


life  histories  of,  95. 

list  of  Waldron,  314. 

lophophore  in  recent,  274. 

mesembryo  of,  248. 

metembryo  of,  248. 

modifications  from  acceleration  in, 

233. 

neanic  period  in,  268. 
neoembryo  of,  249. 
nepionic  period  in  development  of, 

267. 
observations  on,  Lacaze-Duthiers, 

259. 

pedicle-openings  of,  38. 
phylembryo  in,  254. 
post-embryonic  stages  of,  265. 
protegulum  in,  230. 
protembryo  of,  248. 
restraint  of  environment  in,  39. 
revision  of  families  of  loop-bearing, 

290. 

smooth  early  larval  shell  in,  18. 
protegulum  of,  18. 
surface  in  early,  25. 
spine  development  in,  14. 
spines  of,  10. 

Davidson,  51. 

spiniform  structures  in,  77,  78. 
spinose  genera  of,  70. 
spinules  in,  Hall  and  Clarke,  51. 
stages  in  development  of,  292. 
typembryo  of,  250. 
value  of  stages  of  growth  and  de- 
cline in,  Hyatt,  Jackson,  229. 
brachiopods,  initial  shell  in,  255. 
mantle  of,  255. 
mouth  in,  255. 
protegulum  in,  122. 
Waldron,  313. 
Brachyaspis,  146. 
Brachymetopus,  148. 

Brady,  H.  B.,  compound  spines  in  Fora- 
mim'fera,  68. 

pelagic  forms  of  Foraminifera,  66. 
spines  in  Foraminifera,  44,  49. 
terminal  spiniform  processes  in  In- 
fusoria, 45. 
brambles,    general    spininess    of,    88, 

89. 

suppression  of  structures  in,  87. 
Branchipus,  first  pair  of  appendages  of, 


Branco,    W.,    development    of    fossil 
cephalopods,  310. 

study  of  Tornoceras  retrorsum,  var. 

typum,  435. 
Brongniart,   A.,  classification  of  trilo- 

bites,  110,  111. 
Brongniartia,  154. 

glabella  and  pygidium  in,  155. 
Bronteidae,  129,  131,  139. 

defined,  148. 
Bronteopsis,  146. 
Bronteus,  113,  129,  149. 

angusticeps,  glabella  in,  149. 
campanifer,  glabella  in,  149. 
palifer,  glabella  in,  149. 
Brontotheriidae,  extinction  of,  96. 
Brooks,   W.   K.,  arm   development  in 
Glottidia,  274. 

development  of  Lingula,  256. 
growth  of  spiral  arms  in  Glottidia, 

282. 

incipient  stage  of  Glottidia,  386. 
later  larval  stages  of  Glottidia,  246. 
tentacles  in  Glottidia,  274. 
tentacular  multiplication   in  Glot- 
tidia, 407. 
Bryozoa,  spinules  of,  Hall,  45. 

Waldron,  313. 

Buch,  L.  von,  study   of  Tornoceras  re- 
trorsum, 435. 

Bulimina  aculeata,  spines  in,  49. 
Bumastus,  146. 

eyes  in,  147. 

Bunodes,  free-cheeks  in,  118. 
Burmeister,  H.,  classification  of  trilo- 

bites,  111,  112. 
Burmeister ia,  154. 

glabella  and  pygidium  in,  155. 
byssal  plug  of  Anomia,  38. 


Cactaceae  of  America,  leaves  of,  trans- 
formed into  spines,  75. 

cactus,  spines  in,  74. 

Callaway,  C.,  metamorphoses  of  trilo- 
bites,  166. 

young    specimen    of    Ptychoparia 
monile,  178. 

Cattichthys,    male,    spines    in,    Seeley, 
59. 

Callinectes  hastatus,  spines  on,  47. 


INDEX 


605 


Callista  sublamdlosa,    spines    on    post- 

umbonal  slope  of,  45. 
Callopora  exsul,  spines  on,  45. 
Calymmene,  113,  116,  129,  154,207,209. 
facial  sutures  in,  125. 
mouth  in,  Walcott,  209. 
pleural  spines  in,  80. 
Calymmenella,  154. 

glabella  in,  154. 
Calymmenida,  Haeckel,  112. 
Calymmenidse,  129,  131,  139,  152,  153. 

defined,  154. 
Calymmenopsis,  154. 
glabella  in,  154. 
Camarella  group,  245. 
Camarophoria,  245. 
Camarotcechia,  318,  387. 
acinus,  315,  388. 

areal  development  of,  390. 
beak  in,  388. 
discussion  of,  339. 
plications  in,  397. 
sinus  and  fold  in,  388. 
indianensis,  315,  341,  343,  352,  360, 
388. 

discussion  of,  346. 
initial  shell  in,  349. 
plications  in,  397. 
sinus  and  fold  in,  388. 
negkcta,  315, 344, 351,  352,  366,  388. 
abnormal  variation  in,  398. 
areal  development  of,  390. 
beak  in,  388. 
discussion  of,  341. 
plications  in,  397. 
sinus  and  fold  in,  388. 
sobrina,  variation  in,  366. 
Whitii,  315,  360,  366. 

deltidial  plates  in,  390. 
discussion  of,  344. 
plications  in,  397. 
sinus  and  fold  in,  388. 
Cancer  irroratns,  spines  on,  47. 
in  zoe'a  of,  56,  57. 
cannibalistic  selection,  Verrill,  57. 
Caprotina,  236. 
Carausia,  140. 

cephalon  in,  182. 
glabella  in,  140. 
Carcinus,  spines  in  zoe'a  of,  57. 
Cardium,    development    of    spines    in 
many  species  of,  14. 


Carmon,  140. 

glabella  in,  141. 

Carposphcera  melitomma,  spines  in,  48. 
categories  of  origin,  26. 
caterpillar  of    Schizura,    mimicry    in, 

Packard,  62. 

caterpillars  of  certain  moths,  protective 
spines  and  tubercles  in,  Packard,  65. 

origin  of  spines,  etc.,  in,  Packard, 

55. 
cattle  of  southern  Italy,  horns  in,  53. 

spine  production  in,  13. 

Texan,  horns  in,  53. 
Celmus,  148. 

Centaurea,  spinescent  bracts  of,  74. 
Centronella,  245,  283,  291,  305,  306,  411, 
412,  413,  414,  415. 

absence  of  septum  supporting  loop 
in,  298. 

loop  in,  281. 

punctate  shells  in,  411. 
centronelliform  stage,  298. 
Centronellinas,  291. 

defined,  306. 
Centropleura,  142. 
cephalon,  morphology  of,  117. 
cephalopod,  plain  protoconch  of,  18. 
Cephalopoda,  247. 

attenuation  of  the  shell  in,  91. 

secondary  characters  in,  Hyatt,  266. 
cephalopods,    development    of    fossil, 
Hyatt,  Branco,  Mojsisovjcs,  310. 

protoconch  in,  121. 
Ceratiocaris,  three  spines  in,  84. 
Ceratium  fusus,  terminal  spiniform  pro- 
cesses in,  45. 

longicornis,  terminal  spiniform  pro- 
cesses in,  45. 

tripos,  terminal  spiniform  processes 

in,  45. 

Ceratocephala,  151,  152. 
Ceratolichas,  150,  151. 

glabella  in,  150. 

Ceratopsida3,  extreme  spinosity  in,  70. 
Ceratopyge,  142. 
Ceraurus,  113,  116,  155. 

cephalon  and  pygidium  in,  155. 

spiniform  processes  in,  78. 
Cervulus  (?)  dicranoceros,  antler  of,  53. 
Cervus  dama,  antler  of,  53. 

elaphus,  antlers  of,  17. 

pardinensiSf  antler  of,  53. 


606 


INDEX 


chaetopods,  254. 

Chamcdeon  Oweni,  horns  in  male,  59,  60. 

Chamaeleontidse,  horns  in  male,  Darwin, 

59. 
Chapman,  E.  J.,  classification  of  trilo- 

bites,  113. 

characteristics,  inherited,  Hyatt,  100. 
Chariocephalus,  142. 
Chasmops,  129,  156,  157. 

accessory  lobes  in  glabella  of,  157. 
third  segment  in,  127. 
Cheiruridas,  129,  131,  152,  153. 

defined,  155. 
Cheirurus,  129,  155,  156. 

fourth  annulation  in,  127. 

genal  angles  in,  123. 

insignis,  spiniform  processes  in,  78, 

79. 

chilidium,  257. 
Chilomycterus     geometricus,     protective 

spines  in,  55. 
Chonetes,  231,  245,  395. 

absence  of  straight  hinge-line  in 

young,  268. 
nova-scoticus,  315. 
pedicle-opening  in,  241. 
spines  in,  96. 
undulatus,  315. 
cirri,  alternating,  in  adult  lophophore  of 

Brachiopoda,  Hancock,  275. 
Cirripedia,  114. 

respiration  in,  218. 

Cirsium  horridulum,  spiniform  bracts  in, 
75. 

lanceolatum,  spiniform  bracts  in,  75. 
Cistella,  231,  238,  245,  246,  247,  260,  271, 
288,  292,  297,  300,  303,  307,  308,  408, 
409. 

adult  loop  in,  291 . 
archenteron  in,  248. 
arm  development  in,  Kovalevski, 

274. 

attachment  of,  256. 
blastula  cavity  in,  248. 
cephalula  of,  260,  261. 

stage  in,  249. 

cistellula,  adult  lophophore  in,  279. 
deltidial  plates  in,  395. 
development  of,  254. 
difference  in  valves  in,  234,  235. 
early  nepionic  shell  of,  261. 
eyes  in,  249. 


gwyniform  stage  in,  288,  302. 
lophophore  in,  279. 

adult,  275. 
mantle  in,  253. 
mesembryo  of,  248,  249. 
metembryo  in,  248. 
neapolitana,   embryonic  stages  of, 
248,  249,  250. 
Kovalevski,  407. 
lamellae  of  loop  in,  271. 
lophophore  in,  279. 
neoembryo  of,  249,  250. 
nodes  and  ridges  on,  and  pedicle  in, 

305. 

outline  and  hinge  of,  238. 
paragerontic  development  in,  271. 
pedicle  in,  235. 
phylembryo  in,  254. 
protegulum  in,  Kovalevski,  231. 
protembryo  of,  248. 
protembryonic  stages  in,  248. 
researches  on,  Kovalevski,  257. 
schizolophus  stage  in,  278. 
tentacles  in,  Kovalevski,  275. 
in  young  stages  of,  275. 
of  taxolophus  in,  277. 
tentacular  multiplication  in,  Kova- 
levski, 407. 

trocholophus  stage  in,  278. 
typembryo  of,  250,  251,  261. 
variations  in,  78. 
Cladocera,  respiration  in,  218. 
Cladochonus,  425. 
Clarke,  J.  M.,  on  Acidaspis,  151. 

Arges  consanguineus  from  the  Lower 

Helderberg  Group,  174. 
development  of  deltidium,  263. 
development    of    pedicle-opening, 

240. 
gerontic   period    in    Brachiopoda, 

269. 

claspers,  88. 

classification  of  corals,  importance  of 
tabulae  in  a  natural,  Verrill,  427. 
of  trilobites,  109. 
ontogeny  a  standard  of,  L.  Agassiz, 

120. 
principles  of  a  natural,  119, 

Hyatt,  120. 

Claus,  C.,  nauplius  of  Crustacea,  190. 
Clavatula  mitra,  spiniform  prominences 
on,  46. 


INDEX 


607 


Cleiothyris,  dorsal  beak  in,  264. 
Cleistopora  geometrica,  size  and  develop- 
ment of  cells  in,  430. 
climbing  plants,  prickles  on,  90. 
Cliothyris  Royssii,  spinules  in,  51. 
Clitambonites,  245. 

dorsal  fissure  in,  263. 
Clorinda,  245. 

fornicata,  315. 
Ccelospira,  245. 

Barrandei,   brachial    supports    in, 
416. 

disparilis,  369. 

marginalis,    brachial    supports    in, 
416. 

mature  condition  in,  417. 
Ccenothyris,  245,  307. 
Ccerostris    mitralis     mimicking    woody 

excrescence,  Peckham,  61. 
Colivina  pygmcea,  spines  in,  44. 
Columnopora,  428. 
Conchidium,  245. 
Conocephalites,  140. 
Conocoryphe,  129,  139,  140,  153,  178. 

(Bailielld)  Baileyi,  178. 

cephalon  of,  128. 

eye-line  in,  223. 

glabella  in,  141,  182. 
ConocoryphidaB,  128,  129,  131,  139,  140, 
141,  152,  153. 

affinities  of  Harpides  with,  137. 

denned,  140. 
Conolichas,  149,  150. 

glabella  in,  150. 

third  segment  in,  127. 
Conophrys,  133. 
Conotreta,  231,  244. 
Cope,  E.  D.,  bathmic  force  of,  29. 

culmination  and  descent  of  Echino- 
dermata,  26. 

on  variation,  6. 

ossification  of  superior  cranial  walls 

in  toads  and  frogs,  47. 
Copepoda,  114. 

respiration  in,  218. 
Copridse,  protective  horns  in  males  of, 

Wallace,  60. 

coral,  development  of  a  Paleozoic  po- 
riferous, 421. 
corals,  corallites  in  compound,  45. 

external   differentiation   of   struc- 
tures of,  into  spines,  50. 


Paleozoic,  421. 

spines  on  corallites  in  compound, 

Hall,  45. 
Waldron,  313. 

Corda,  A.  J.  C.,  classification  of  trilo- 
bites,  111. 

generic  and  specific  names  for  dif- 
ferent stages  of  growth  of  Sao 
hirsuta,  166. 
generic  names  for  forms  of  Agnos- 

lus,  136. 
Cordania,  148. 

fixed-cheeks  in,  148. 
Cornulella  hexagona,  terminal  spine  in, 

44. 
Coronura,  156,  157. 

accessory  lobes  in  glabella  of,  157. 
diurus,  157. 
myrmecophorus,  157. 
Corycephalus,  156,  157. 

notched  cephalon  in,  157. 
Corynexochus,  142. 
cow,  growth  of  horns  of,  10. 
crab,  common,  zoe'a  of,  56. 

horse-shoe,  development  of  anterior 
legs  in,  84. 
spines  in,  22. 
tail  spine  in,  84. 
crabs,  cephalothorax  in,  84. 
Crania,  231,  239,  244,  284,  387. 
anal  opening  in  recent,  264. 
arms  of,  276. 

difference  in  valves  in,  234,  235,  236. 
pedicle-opening  in,  241. 
setifera,  314. 
siluriana,  314. 

discussion  of,  317. 
spinigera,  314. 
tentacles  in,  275. 
Cranias,  Waldron,  313. 
Craniella,  231,  244. 
Craniscus,  244. 
Crepicephalus,  142,  145. 
Ceratopyge,  145. 
Crinoidea,  compound  spines  in,  68. 

development  of  tubercles  of,  into 
spines,  Wachsmuth  and  Springer, 
50. 

immature  forms  of,  Waldron,  313. 
smooth  surface  in  early,  25. 
spines  on  ventral  sacs  of,  Wachs- 
muth and  Springer,  45. 


608 


INDEX 


crinoids,  Waldron,  313. 
Crioceras,  96. 

Cristellaria  calcar,  spines  in,  49. 
Cromus,  153. 

Crotalocephalus,  153,  155,  156. 
fourth  annulation  in,  127. 
Crotalurus,  148. 

Crucibulum  spinosum,  spines  in,  51. 
Crustacea,  157,  167,  168,  169,  170,  187, 
188,  189,  190,  193,  194,  202,  209,  210, 
215,  219. 

anal  segment  of,  216,  217. 
aquatic,  suppression  of  appendages 

in,  83. 

evolution  among,  36. 
intimate  relationship  of  trilobites 

with,  110. 
modern,  186. 
nauplius  of,  170,  205. 
Claus,  190. 
modern  higher,  121. 
principle  of  cephalization  in,  Dana, 

83. 

protective  spines,  in,  55. 
protonauplius  form  of,  121. 
rank  of  trilobites  in,  114. 
reduction  in  limbs  of,  83. 
results  of  hypertrophy  in,  Packard, 

38. 
second  theoretical  head  segment  in, 

119. 

spineless  nauplius  of,  19. 
spine  production  in,  13. 
spines  in,  10. 

in  larval  stages  of,  19. 
spiniferous  ridges  on,  52. 
spinous  prominences  on  test  of,  9. 
stalked  eyes  in,  117,  119. 
structures  in  higher,  117. 
Trilobita  the  earliest  forms  of,  185. 
trilobites  an  appendage  to,  Lang, 
114. 

a  sub-class  of,  Kingsley,  114. 
classed  with,  109. 
crustacean,  form  of  original,  189. 

head,  development  of,  123. 
crustaceans,  amphipod,  found  in  lakes 
Baikal  and  Titicaca,  64. 

Waldron,  313. 
Crypkceus,  156. 

plain  cephalon  in,  157. 
Cryptondla,  245,  291,  305,  307. 


Cryptonymus,  146. 
Cryptopora,  231,  245. 

protegulum  in,  234. 
Ctenocephalus,  140,  153,  178. 

glabella  in,  141,  182. 

(Hartelld)  Matihewi,  small  cephalon 

of,  Matthew,  178. 
Ctenopyge,  142. 
Cumacea,  200. 
cuticles  in  embryonic  stages  of  insects, 

19. 

Cyathaxonia  cynodon,  spinules  in,  50. 
Cyathophycus  reticulatus,  49. 
Cybele,  153. 

eye-line  in,  154. 

pygidium  in,  223. 
Cyclognathus,  142. 
Cyclops,  189. 

fourth  pair  of  limbs  of,  192. 

nauplius  of,  164. 
Cydopyge,  146,  188. 
Cyphaspis,  113,  141,  148. 

eyes  in,  148. 

fixed-cheeks  in,  148. 

glabella  in,  137. 
Cyphoniscus,  133. 
Cypris,  36. 

bivalve  shell  in,  193. 

nauplius  in,  192. 
Cyrtia,  385. 
tiyrtina,  245,  385. 
Cyrtometopus,  155. 

direct  line  from,  Reed,  156. 

pentamerous  lobation  of  glabella  in, 

155. 

Cystoidea,  smooth  surface  in  early,  25. 
Cytisus  spinosus,  spiniform  branches  of, 
75. 


Dcemonorops  hygrophilus,  leaf  of,  89. 
Daikaulia,  242. 

Dall,  W.  H.,  characters  of  Thecidium, 
256. 

classification  of  Brachiopoda,  290. 
earliest  stages  of  development  of 

Terebratalia  obsoleta,  297. 
loop  in  Leiothyrtna  cubensis,  396. 
Terebratella  occidentalis,  var.   obso- 
leta, 406. 
Dallina,  288,  297,  299,  301,  303,  306,  308. 


INDEX 


609 


adult  loop  in,  291. 
Jloridana,  297,  300. 
Grayi,  297. 

adult  characters  of  loop  in,  295. 
ismeniform  stage  of,  300. 
muhlfeldtifortn  stage  in,  301. 
new  genus,  297. 
Raphaelis,  297. 

adult  characters  of  loop  in,  295. 
septigera,  300,  409. 

adult  characters  of  loop  in,  295. 
brachial  supports   in,    Friele, 

Deslongchamps,  406. 
development  of  loop  in,  Friele, 

Deslongchamps,  292. 
transformations  in,  Friele,  298. 
Dallinime,  295,  302. 

boreal  distribution  of,  304. 
denned,  307. 
median  septum  in,  296. 
ontogeny  and  morphology  of,  300. 
small  cardinal  processes  in,  296. 
stages  of  growth  in,  288,  303. 
Dalman,  J.  W.,  classification  of  trilo- 

bites,  111. 
Dalmanella,  387,  388,  394. 

absence  of  embryonal  sinus  in,  388. 
elegantula,  315,  317,  318,  321,  322, 
352. 

discussion  of,  317. 
earliest  growth-stages  in,  373. 
mature  individuals  in,  373. 
Dalmanites,  113,  129,  156,  157,  179. 
eyes  in,  126. 

features  of  dorsal  shield  of,  182. 
free-cheeks  in,  117,  124,  181. 
genal  angles  in,  123. 
larvae  of,  128. 

nasutus,  spinose  cephalon  in,  157. 
no  eye-line  in,  180. 
ontogeny  of,  128,  129,  135. 
pleura  from  pygidium  in,  127. 
pygidium  in,  181. 
socialis,  175,  176. 
tridens,  spinose  cephalon  in,  157. 
Dana,  J.  D.,  principle  of  cephalization 

in  Crustacea,  83. 

Daphnia,  absence  of    median  eye   in, 
193. 

nauplius  appendages  of,  192. 
Darwin,  C.,  horns  in  Chamseleontidae, 
59. 


origin  of  so  called  rudimentary  or- 
gans, 80. 

sexual  selection  as   affecting  the 
growth  of  antlers  in  the  deer,  58. 

spines  on  shells  of  barnacles,  52. 
Davidson,  T.,  casts  of  pedicles  of  fossil 
Lingula3  and  Eichwaldia,  257. 

relationsof  Bilobiteswifh  Orthis,3S9. 

spines  in  Brachiopoda,  51. 
Davidsonella,  245. 

schizolophus  stage  in,  279. 
Davidsonia,  245,  284. 

hinge  in,  236. 

spiral  impressions  on  valves  of  276. 
Daviesiella,  245. 
Dayia,  245,  413,  416. 

navicula,  413. 
Decapoda,  200. 

spines  on  zoe'a  of,  46,  57. 

spiny  forms  of,  70. 
Dechenella,  148. 

lobation  of  glabella  in,  148. 
deer,  absence  of  antlers  in  ancestors  of, 

24. 

antlers  of,  9,  21. 

development  of  horns  in,  14. 

early,  antlers  in  both  sexes  of,  57. 

fallow,  antler  of,  53. 

family,  compound  antlers  in,  68. 

growth  in  antlers  of,  10. 

modern,  shedding  of  antlers  of,  97. 

sexual  selection  as  affecting  growth 
of  antlers  in,  Darwin,  58. 

simple  antlers  of  young,  24. 

spine  production  in,  13. 

Tertiary,  simple  antlers  in,  52. 
defences,  mechanical  and  motor,  in  ani- 
mals and  plants,  Morris,  97. 
Deiphon,  153,  155,  156. 

Forbesi,  spiniform  processes  in,  78, 
79. 

free-cheeks  in,  118. 

glabella  and  pygidium  in,  155. 
Delthyris,  399. 
delthyrium,  257. 
deltidial  plates,  origin  of,  257. 
deltidium  amplectens,  391. 

development    of,    Deslongchamps, 
Clarke,  263. 

discretum,  391. 

origin  of,  257. 

sectans,  391. 


610 


INDEX 


Derbya,  245. 

Deslongchamps,  E.,  brachial  supports 
in  Macandrevia  cranium,  Dallina  septi- 
gera,  and  M'uhlfddtia  sanguinea,  406. 
characters  of  pedicle-openings,  240. 
classification  of  Brachiopoda,  290. 
deltidium  in  brachiopods,  391. 
development  of  the  deltidium,  263. 
of  loop  in  Macandrevia  cranium 

and  Dallina  septigera,  292. 
platidiform  stage  in  Platidia,  298. 
desmids,  spine  growth  in,  42,  43. 
Desmoncus  polycanthus,  leaf  of,  89. 
development  of  trilobites,  109. 
Diaphanometopus,  155. 

pentamerous  lobation  of  glabella  in, 

155. 

diatoms,  spine  growth  in,  43. 
Diccdosia,  King,  399. 
Dicranogmus,  150. 

glabella  in,  150. 
Dicranograptus     Nicholsoni,     apertural 

spines  on,  45. 
Dicranurus,  113,  151,  152. 
Dictyocephalites,  140. 
Dictyondla,  absence  of  embryonal  sinus 
in,  388. 

Capewelli,  336. 
reticulata,  315,  388. 
beak  in,  388. 
discussion  of,  335. 
Dictyospongia  Conradi,  49,  50. 

prismatica,  50. 

Dictyospongidse,  progressive  differentia- 
tion of  ornament  in,  Hall,  49. 
Dictyothyris,  245,  305,  307. 
Dielasma,  245,  283,  291,  305,  307,  411, 
412. 

brachial  supports  in,  276. 
Centronella-like  loop  in,  281. 
development  of  brachial  supports 

in,  410. 

loop  in,  411,413. 
turgidum,  brachidium  in,  281. 

development  of  brachial  sup- 
ports in,  410. 

loop  in,  412. 
Dielasmina,  307. 
differentiation,  specific,  in  Achatinellse, 

Hyatt  and  Verrill,  36. 
Difflugia  acuminata,  spines  on  fundus  of, 
43. 


with  spine  from  fundus,  44. 

constricta,  spines  in,  44. 
variable  form  of,  43. 

corona,  spines  on  fundus  of,  43. 

globulosa,  spherical  shell  of,  43. 

pyriformis,  pear-shaped  shell  of,  43. 
Dignomia,  243. 
Dikelocephalinae,  145. 
Dikelocephalus,  113,  142,  143,  145. 
Dimerella,  245. 
Dindymene,  153. 

glabella  in,  154. 

pentamerous  head  axis  in,  154. 

pygidium  in,  223. 

spiniform  processes  in,  78. 
Dinobulus,  243. 

Dinocerata,  no  descendants  of,  96. 
dinosaur,  Jurassic,  suppressed  first  digit 

in,  85. 

Dinosauria,    gigantic    Cretaceous,    ex- 
treme spinosity  in,  70. 

great  horned,  protective  plates  and 

spines  in,  54. 
Diodon,  70. 

maculatus,  protective  spines  in,  55. 
Dionide,  137,  138,  220. 

determination  of  sutures  in,  Bar- 
rande,  134. 

eye-spots  lost  in,  135. 
Dipleura,  129,  154. 

glabella  and  pygidium  in,  155. 
Discina,  231,  244. 
Discinidae,  287. 
Discinisca,  231,  244,  247,  255,  268,  284. 

arms  of,  276. 

difference  in  valves  in,  234,  235. 

discoid  form  of  shell  in,  287. 

growth  of  spiral  arms  in,  Muller, 
282. 

hinge-line  lost  in,  236. 

larval  form  of,  Muller,  246. 

outline  and  hinge  in,  238. 

pedicle  in,  235,  251. 
-opening  in,  241. 

protegulum  in,  229,  234. 

recent,   discoid  form  of    shell  in, 
287. 

similar  form  in  both  valves  in,  236. 

stages  of  development  in,  265,  266. 

striata,  236. 

tentacles  in,  275. 

of  taxolophus  in,  277. 


INDEX 


611 


Discinopsis,  244. 

Disculina,  245. 

dissoconch  growth  in  Avicula,  23. 

distortions,  pathologic,  Hyatt,  40. 

divergence,  reproductive,  Vernon,  37. 

Dolichometopus,  146. 

Dorycrinus  unicornis,  spines  on  terminal 

sacs  of,  45. 
Dorypyge,  142. 
Dybowsky,  B.  N.,  specific  development 

of  Gammarus  in  Lake  Baikal,  65. 
Dynastes  hercules,  horns  in,  59. 
Dynastidse,  protective  horns  in  males 

of,  Wallace,  60. 
Dyscolia,  278,  307. 

absence  of  septum,  hinge-plate,  and 
dental  plates  in,  278. 

adult  arm  structure  in,  291. 

discoid  lophophore  in,  Fischer  and 
CEhlert,  278. 

loop  in,  282. 

trocholophus  stage  in,  278. 
Dyscoliidae,  307. 
Dyscoliinse,  291. 

defined,  307. 
Dysplanus,  146. 

eyes  in,  147. 

position  of  eyes  in,  147. 


E 

Eatonia,  245. 
Eccoptocheile,  129,  155. 

pentamerous  lobation  of  glabella  in, 

155. 
Echidna,  extreme  spinosity  in,  70. 

male,  spur  on  hind  limbs  of,  9,  59. 
modified  hairs  of,  9. 
protection  of,  by  spines,  54. 
Echidnoceras,  70. 

setimanus,  spiny  character  of,  55. 
Echinaster  spinosus,  protective  spines  in, 

56. 
Echinocaris  socialis,  three  spines  in,  84, 

85. 
Echinodermata,  25,  230. 

a  typically    spiniferous    sub-king- 
dom, 68. 

generally  movable  processes  of,  9. 
homologous  structures  in,  94. 
localized  stages  of  growth  in,  Jack- 
son, 16. 


movable  spines  of,  94. 

primitive  type  of  structure  in,  26. 

repetition  of  structures  in,  68. 
Echinoidea,  compound  spines  in,  68. 

minute  spines  in  early  genera  of,  25. 

spine  growth  in,  69. 

spine  production  in,  13. 
echinoids,  protechinns  in,  1 22. 
Echinops,  foliage  of,  transformed  into 

spines,  74. 
EctillcBnus,  146. 

position  of  eyes  in,  147. 
Edestus  vorax,  spines  of,  68. 
Edwards  and  Haime,  spinules  in  septal 
f?  lines  of  Tetracoralla,  50. 
effects,  summary  of  the  operation  of  the 

law  of  multiplication  of,  6. 
eggs,  possible  trilobite,  169. 
Eichwaldia,  casts  of  pedicle  of,  David- 
son, Walcott,  257. 

diagnosis  of,  Billings,  336. 

reticulata,  335. 
elk,  antlers  of,  9. 

simple  antlers  of  young,  24. 

spine  production  in,  13. 
Elkania,  243. 

Ellipsocephalus,  142,  143,  145. 
Elliptocephala,  142,  143,  144,  145. 

asaphoides,  144. 

interocular  spines   of  young, 

136. 

pleura  from  the  glabella  in,  127. 
pygidium  in,  143. 
retrally  directed  pleura  of,  79. 
Emmrich,  H.  F.,  classification  of  trilo- 

bites,  111,  112. 
Encrinuridae,  129,  131,  152,  153. 

defined,  153. 
Encrinurus,  113,  129,  153. 

eye-line  in,  154. 

glabella  in,  154. 

pygidium  in,  223. 
Endesia,  297. 

cardium,  297. 
endopodites  of  Triarthrus,  224. 

of  Trinucleus,  224. 
Endymionia,  138,  220. 
energy,  bathmic,  34. 

entergogenic,   effect  of  action  of, 
Hyatt,  29. 

of  growth  force,  31,  34,  38. 
Enteletes,  245. 


612 


INDEX 


Entomolithus  paradoxus,  111. 
Entomostraca,  114,  115,  202. 

affinities  of  trilobites  with,  164. 
environment,    external    stimuli    from, 

31. 
restraint  of,  in  some  Brachiopoda 

and  Trilobita,  39. 

Epeira  spinea,  protective  mimetic  fea- 
tures in,  62. 
Epeiridae,  protective  mimetic  features 

in,  62. 

ephebic  period  in  Brachiopoda,  269. 
stage,  248. 

of  trilobites,  121. 
Erinnys,  140. 
Eryx,  140. 

glabella  in,  141. 
Eucalathis,  291,  307. 

zugolophus  stage  in,  281. 
Eucrustacea,  114. 
Eucyrtidium  elegans,  terminal  spine  in, 

44. 
Eudesella,  245,  284. 

digitata,  lophophore  in,  280. 
mayale,  lophophore  in,  280. 
Eudesia,  245,  306. 
Euglypha  mucronata,  terminal  spine  on, 

43. 

Euloma,  142. 
Eumetria,  245. 
Euphausia,  185,  186,  187. 

beginnings  of  metastoma  in,  192. 
Euphorbiaceas  of  Africa  and  southern 
Asia,    leaves    of,    transformed    into 
spines,  75. 
Eurycare,  142,  145. 
Eurypterus,  tail  spine  in,  84. 
Eutrilobita,  Haeckel,  112. 
evolution  among  Crustacea,  36. 
general,  3. 
progressive,  Agassiz,  28. 

Cope,  28. 

exopodites  of  Trinucleus,  224. 
eyes,  schizocroal,  in  Phacopidse,  157. 


Fagonia,  spiniform  stipules  of,  74. 
Favosites,  424,  427,  433. 

budding  in,  426. 

buds  in,  432. 


Forbesi,    var.    occidentalis,    young 

colony  of,  425. 
mural  pores  in,  427. 
origin  and  development  of,  425. 
size  of  corallites  in,  429,  430. 
spinigerus,  spines  on,  45. 
Favositidas.  symmetrical  cell  develop- 
ment in,  429. 

Faxon,  W.,  evolution  of  Allorchestes  in 
Lake  Titicaca,  36. 

wonderful  specific  development  of 
Allorchestes  in  Lake  Titicaca,  65. 
Fewkes,  J.  W.,  development  of  Spiror- 
'     bis,  253. 

Fischer,  P.,  and  CEhlert,  D.-P.,  brachial 
supports  in  Terebratella  dorsata  and 
Magellania  venosa,  from  Tierra  del 
Fuego,  406. 

Brachiopoda,  290. 
centronelliform  stage,  298. 
development  of  the  loop  in  Terebra- 
tella dorsata,  293. 

discoid  lophophore  in  Dyscolia,  278. 
stages  in  development  of  Brachiop- 
oda, 292. 

fishes,  compound  spines  of,  68. 
protective  characters  of,  55. 
spines  of,  9. 

spiniferous  lines  and  ridges  on,  52. 
spiny  forms  of,  70. 
flora,  desert,  86. 

of  arid  regions,  Henslow,  71. 
Foraminifera,     compound     spines     in, 
Brady,  68. 

pelagic  forms  of,  Brady,  66. 
spines  in,  44. 

Brady,  49. 
force,  bathmic,  Cope,  29. 

deficiency  of  growth,  31,  39,  41. 
energy  of  growth,  31,  34,  38. 
nerve,  32. 
thought,  32. 

forces  affecting  growth,  31. 
Ford,  S.  W.,  development  of  trilobites, 
310. 

metamorphoses  of  trilobites,  166. 
young  of  Olenellus  asaphoides,  178. 
form  in  Brachiopoda,  genesis  of,  238. 
Friele,  H.,  brachial  supports  in  Macan- 
drevia  cranium,  Dallina  septigera,  and 
Muhlfeldtia  sanquinea,  406. 

development  of  Brachiopoda,  290. 


INDEX 


613 


of  loop  in  Macandrevia  cranium 

and  Dallina  septigera,  292. 
transformations  in  Dallina  septigera, 

298. 

frustule  of  Attheya  decora,  spine  growth 
from,  43. 

of   Staurastrum  cuspidatum,    spine 

growth  from,  43. 

of  Xanthidium  armatum  and  Anthro- 
desmus  octocornis,  spiue  growth 
from,  43. 

fundus  of  Difflugia  acuminata,   spines 
on,  43. 

corona,  spines  on,  43. 
constricta,  spines  on,  43. 
Fusus  colus,  spiniform  prominences  on, 
46. 


Gammarus,  evolution  of,  in  Lake  Baikal, 
Jackson,  36. 
specific  development  of,  in  Lake 

Baikal,  Dybowsky,  64,  65. 
Gasteropegmata,  242. 
Gastropoda,  conical    non-coiled,  spine 
development  in,  14. 

development  of  spines  in,  14. 
immature  forms  of,  Waldron,  313. 
protoconch  in,  122. 
spines  in,  Hall,  51. 

young,  57. 

spiniform  prominences  on,  46. 
gastropods,  Waldron,  313. 
Gecarcinus  ruricola,  spiniferous  ridges 

on,  52. 

Geddes,  P.,  decline  of  vitality  in  thorny 
plants,  97. 

on  ebbing  vitality,  12. 
Gegenbaur,  C.,  classification   of    trilo- 
bites,  114. 

functions  of  respiration  and  loco- 
motion, 218. 
Gelasimus  princeps,   spiniferous   ridges 

on,  52. 
genetic  sequence  in  forms  of  Brachiop- 

oda,  24. 
geometrid  moths,  spines  on  larvae  of, 

Packard,  46. 
Gerasaphes,  146. 

gerontic  stage  of  brachiopods,  248. 
of  trilobites,  121. 


Gilbertsocrinus  tuberosus,  spines  in,  50. 
giraffe,    accelerated     development    of 
horns  in,  22. 
born  with  horns,  19. 
production  of  horns  in,  13. 
Glassia,  245,  416. 
Globigerina  bulloides,  pelagic  forms  of, 

66. 

Glossina,  243. 
Glossothyris,  307. 
Glottidia,  231,  243,  247,  255,  256. 
albida,  nepionic  stage  in,  267. 
arm  development  in,  Brooks,  274. 
growth  of  spiral  arms  in,  Brooks, 

282. 

incipient  stage  of,  Brooks,  386. 
later  larval  stages  of,  Brooks,  246. 
lophophore  in,  282. 
pedicle  in,  251. 
protegulum  in,  234. 
pyramidata,  lophophore  in,  282. 
schizolophus  stage  in,  278. 
tentacles  in,  274,  275. 
tentacular  multiplication  in,  Brooks, 

407. 

trocholophus  stage  in,  278. 
Goldfuss,  A.,  classification  of  trilobites, 

111. 

Goldius,  149. 
Goniatites  discoideum,  436. 

uniangulare,  435. 
Gould  and  Pyle,  ichthyosis,  92. 
graptolites,  apertural  spines  on,  Nichol- 
son and  Lydekker,  45. 
Graptolithus   quadrimucronatus,    apertu- 
ral spines  on,  45. 
Gratacap,  L.  P.,  numerical  intensity,  35. 

Zoic  maxima,  35. 

Gray,  A.,  summer  shoots  of  barberry, 
12. 
J.  E.,  classification  of  Brachiopoda, 

290. 

Griffithides,  148. 
eyes  in,  148. 
fixed-cheeks  in,  148. 
glabella  in,  148. 

growth,  conditions  or  forces  affecting, 
31. 
force,  deficiency  of,  31,  39,  41. 

energy  of,  31,  34,  38. 
localized  stages  of,  14. 
Jackson,  16. 


614 


INDEX 


of  a  spine,  direct  and  progressive,  10. 

indirect  and  regressive,  10. 
Grunewaldtia,  245. 
guinea  pigs,  growth  in,  Minot,  92. 
Giinther,  A.,  mimetic  features  of  Aus- 
tralian pipe-fish,  63. 

protective  characters  of  Australian 

pipe-fish,  55. 

Gwynia,  231,  271,   278,  288,  289,  292, 
297,  299,  307,  308,  407,  409. 
absence  of  calcareous  loop  in,  271. 

deltidial  plates  in  adult,  261. 
adult  loop  in,  291. 
labial  appendages  in,  King,  271. 
no  metamorphoses  in,  302. 
paragerontic  development  in,  270. 
tentacles  in,  275. 

to  Dallina,  morphogeny  from,  303. 
to  Magellania,  morphogeny  from, 

303. 

to  Megathyris,  morphogeny  from, 
302. 


Haeckel,  E.,  account  of  the  Lambert 
family,  92. 

classification  of  the  articulates,  112. 
law  of  radial  symmetry,  236. 
report  on  Radiolaria,  16. 
spines  in  Radiolaria,  48. 

in  Spumellarian  Radiolaria,  44. 
hairs,  modified,  of  Echidna  and  Porcu- 
pine, 9. 

Hall,  J.,  description  of  Proetus  parvius- 
culus,  175. 

dorsal  foramen  in  some  Brachi- 

opoda,  264. 

progressive  differentiation  of  or- 
nament in  Dictyospongidae,  49. 
spines  in  Gastropoda,  51. 

on   corallites   in  compound 

corals,  45. 

spinules  of  many  Bryozoa,  45. 
and  Clarke,  J.  M.,  compound  spines 
in  Spirifer  hirtus,  68. 

development  of  spinose  forms 

of  Brachiopoda,  24,  25. 
inception  of  Lingula,  286. 
marginal     spines    of    Atrypa 
hystrix,  45. 


spinules  in  Brachiopoda,  51. 
,  411. 

brachial  supports  in,  413. 

Nicoletti,  short  spire  of,  413. 

recurved  loop  in,  413. 

Saffordi,  short  spire  of,  413. 
Hamites,  96. 

Hancock,  A.,  alternating  cirri  in  adult 
lophophore  of  Brachiopoda,  275. 

fleshy  portion  of  the  arms  in  Lio- 

thyrina  and  Terebratulina,  292. 
Harpedidae,  130,  131,  134,  139. 

defined,  137. 

free  segments  in,  137. 

primitive  head  structure  in,  135. 
Harpes,  113,  130,  137,  222. 

cephalon  in,  127,  137. 

determination  of  sutures  in,  Bar- 
rande,  134. 

d'Orbignyianum,  glabella  in,  137. 

eye-line  in,  223. 
-spots  in,  135. 

fourth  annulation  in,  127. 

free-cheeks  in,  124. 

hypostoma  in,  135. 

lines  on  cephalon  of,  118. 

ocular  ridges  in,  222. 

pygidium  in,  137. 

triangular  areas  in  adult,  222. 

ungula,  glabella  in,  137. 
Harpides,  137,  220,  222. 

affinities  of,  with  Conocoryphidse, 
137. 
with  Harpes,  137. 

ocular  ridges  in,  222. 
Harpina,  137,  220. 
Harris,  G.  F.,    taxonomic     value     of 

spines,  101. 
hart,  royal,  17. 
Hartella  Matthewi,  small  cephalon  of, 

Matthew,  178. 
Harttia,  140. 

glabella  in,  141. 
Hausmannia,  156. 

hawthorn,  spiniform  branches  of,  75. 
Heider,  law  of  radial  symmetry,  236. 
Helicopegmata,  242,  270,  412,  416,  417. 
Heliodiscus  astericus,  spines  in,  48. 

cingillum,  spines  in,  48. 

glyphodon,  spines  in,  48. 
Heliodrymus  dendrocyclus,  spines  in,  48. 
Helmersenia,  244. 


INDEX 


615 


Hemiaspis,  studies  in,  118. 
Hemipronites,  245. 
Hemisphcerocoryphe,  155,  156. 
Hemithyris,  231,  245. 

adult  brachia  in,  282. 
psittacea,  adult  brachia  in,  282. 
Henslow,    G.,  cultivation    of    Acantho- 
sicyos  horrida,  74. 

differentiation  of  parts  of    desert 

plants  into  spines,  74. 
flora  of  arid  regions,  71. 
origin  of  xerophilous  plants,  74. 
spiny  varieties  of  plants,  74. 
Hindella,  245. 
Hinnites,  23. 
Hipparionyx,  245. 
Hippurites,  hinge-line  lost  in,  236. 
hoactzin,  change  in  fore  limb  of,  Lucas, 
84. 

of  South  America,  thumb  and  in- 
dex finger  in,  84. 
Holasaphus,  146. 
Holmia,  113,  142,  143,  144. 
facial  sutures  in,  143. 
interocular  spines  of,  136. 
Kjerulfi,  144. 
pygidium  in,  143. 
retrally  directed  pleura  of,  79. 
Holocephalina,  146. 

position  of  eyes  in,  147. 
small  eyes  in,  147. 
Holometopus,  133. 
Homalonotus,  113,  133,  154. 
DeKayi,  134. 
delphinocephalus,  134. 
free-cheeks  in,  124. 
glabella  and  pygidium  in,  155. 
Knighti,  134. 
Homalopecten,  146. 
Homalops,  156. 

accessory  lobes  in  glabella  of,  157. 
Homarus,  development  of  tail  in,  83. 

spines  in  zoe'a  of,  57. 
Homceospira  evax,  315,  351,  352,   356, 
367. 

brachial  supports  in,  395. 

brachidium  of,  284. 

circular  apical  perforation  in, 

389. 

deltidial  plates  in,  390. 
discussion  of,  360. 
internal  apparatus  of,  365. 


loop  in,  396. 
sobrina,  315,  360. 

discussion  of,  366. 
plications  in,  397. 
sinus  and  fold  in,  388. 
Homolichas,  150. 

glabella  in,  150. 
Hoplolichas,  150,  151. 

extremes  of  spinosity  in,  56. 
glabella  in,  150. 
horn,  definition  of,  9. 
horns,  development  of,  in  deer,  14. 
in  horned  ungulates,  47. 
in  ox,  14,  21. 

for  protection  and  offence  in  hol- 
low-horned mammals,  54. 
in  cattle  of  southern  Italy,  53. 
in  Chama3leontida3,  Darwin,  59. 
in  giraffe,  19. 
in  musk-ox,  53. 
in  Protoceras,  Marsh,  58. 
in  Kuminata,  58. 
in  some  Hercules  beetles,  Bateson, 

59. 

in  stag  beetle,  60. 
in  Texas  cattle,  53. 
sexual  variations  of,  57. 
variations    in    the    directions    of, 

Marsh,  54. 

horseshoe-crab,  spines  in,  22. 
Huxley,  T.  H.,  Articulata  and  Inarticu- 

lata,  242. 
Hyatt,  A.,  Achatinellse,  36. 

Baculites  a  typical    phylogerontic 

form,  91. 
classification  of  stages  of  growth 

and  decline,  247,  248. 
development  of  fossil  cephalopods, 

310. 

early  stages  of  ontogeny,  95. 
entergogenic  energy,  29. 
evolution  of    Tertiary   species    of 

Planorbis  at  Steinheim,  65. 
genesis  of  the  Arietidae,  247. 
gerontic  stages  in  Brachiopoda, 

398. 

inherited  characteristics,  100. 
law  of  morphogenesis,  229. 
nomenclature  proposed  by,  247. 
pathologic  distortions,  40. 
pathological  varieties  of  Steinheim 
Planorbis,  91. 


616 


INDEX 


principles  of  a  natural  classification, 

119,  120. 

secondary    characters    in    Cephal- 
opoda, 266. 

value  of  stages  of  growth  and  de- 
cline in  Brachiopoda,  229. 
Hydnoceras  nodosum,  50. 
phymatodes,  50. 
tuberosum,  49,  50. 
Hydrocephalus,  143,  177,  178. 

car  ens,  177. 
Hydrolenus,  146. 

Hymenactura  copernici,  spines  in,  44. 
hypertrophy  a  result  of  abundant  nu- 
trition, 64. 

in  Crustacea,  results  of,  Packard, 38. 
Hypoparia,  124,  129,  132,  134,  139. 

defined,  134. 
hypostoma  of  Triarthrus,  208. 


ichthyosis,  Gould  and  Pyle,  92. 
Iguanodon     bernissartensis,    suppressed 

first  digit  in,  85. 
Illsenidse,  146. 
Illcenopsis,  146. 

eyes  in,  147. 
Illcenurus,  146. 
Illainus,  129,  146. 

eyes  in,  147. 

free-cheeks  in,  1 24. 

(Octillcenus)     Hisingeri,    spiniform 
pleural  extremities  of,  79. 

pleural  spines  in,  80. 

smooth  glabella  in,  126. 
Inarticulata,  anal  opening  in,  264. 

Huxley,  242. 
Infusoria,  terminal  spiniform  processes 

in,  45. 

Insecta,  Packard,  38. 
insects,  mimetic  forms  of,  61. 

reduction  in  limbs  of,  83. 

simple  cuticles  in  embryonic  stages 
of,  19. 

spines  in  larval  stages  of,  19, 

spinous  prominences  on  test  of,  9. 
intensity,  numerical,  Gratacap,  35. 
Iphidea,  232,  233,  243,  267,  286,  287. 

possible    absence  of    trocholophus 
stage  in,  278. 


taxolophian   brachial  structure  in 

adult,   277. 

Ismenia,  245,   288,  300,  301,  303,  304, 
306,  307,  308. 
adult  loop  in,  291. 
attachment  of  loop  in,  292. 
gwyniform  stage  in,  302. 

and  cistelliform  stages  in,  288. 
ismeniform  stage,  298. 
Isocolus,  133. 

Isopoda,  simple  antennule  in,  205. 
Isotelus,  146. 
itch-mite,  suppression  of  limbs  in,  83. 


Jackson,  R.  T.,  classification  of  stages 
of  growth  and  decline,  248. 

evolution    of    Gammarus  in   Lake 

Baikal,  36. 

law  of  radial  symmetry,  236. 
localized  stages  of  growth,  16. 
observations  made  upon  the  oyster 

and  its  allies,  237. 
phylembryo  in  trilobites,  121. 
phylogeny  of  the  Pelecypoda,  23. 
prodissoconch  of  pelecypods,  231. 
spines  in  Mollusca,  51. 
value  of  stages  of  growth  and  de- 
cline in  Brachiopoda,  229. 
Joubin,  L.,  brachiopods  an  independent 

class,  254, 
Juvavella,  306,  411. 


K 


Kampylopegmata,  242,  290. 
Karpinskya,  245. 
Kayseria,  245,  283. 
Kayserlingia,  244. 

Kerner  von  Marilann,  A.,  barbed  spines, 
etc.,  on  thicket  plants,  88. 

leaves  in  a  tropical  Bignonia,  89, 

90. 
length  of  stems  in  climbing  palm, 

88. 
number    of    species    of    climbing 

plants,  87. 
kinetogenesis,  6. 
King,    W.,    dorsal    foramen    in    some 


INDEX 


617 


Brachiopoda,  264. 
labial  appendages  in  Gwynia,  271. 
Terebratellidae,  292. 
the  genus  Diccelosia,  399, 
Kingena,  245,  304,  306,  308. 
Kingsley,  J.  S.,  affinities  of  trilobites, 
110. 

composition  of  trilobite  head,  188. 
extinction  of  dinosaurian  reptiles, 

96. 

trilobites  as  a  sub-class  of  the  Crus- 
tacea, 114. 
Koninck,  L.  de,  spines  on  Michelinia 

favosa,  39. 
Koninckella,  245. 
Koninckina,  245,  270,  283. 
Koenigia,  154. 

Korshelt,  law  of  radial  symmetry,  236. 
Kovalevski,  A.  O.,  arm  development  in 
Cistella  and  Thecidea,  274. 
early  embryology  of  brachiopods, 
246. 

sedentary  larvae  of  Thecidium, 

256. 

protegulum  in  Cistella,  231. 
tentacles  in  Cistella,  275. 
tentacular  multiplication  in  Cistella 

and  Lacazella,  407. 
ventral  valve  in  Thecidium,  259. 
Kraussina,  231,  238,  245,  294,  295,  308. 
deltidial  plates  in,  395. 

and  thickened  septum  of,  305. 
incomplete  secondary  loop  in,  301 . 
paragerontic  tendency  in,  271. 
plectolophous  stage  in,  281. 
zugolophous  stage  in,  281. 
Kutorgina,  232,  233,  268. 
cingulata,  233,  252. 
deltidium  in,  253. 
Labradorica,  233. 
rudimentary  teeth   in,  Schuchert, 

252. 

sculptilis,  233. 
Whxtfiddi,  233. 


Lacaze-Duthiers,  H.,  early  embryology 
of  brachiopods,  246. 

sedentary  larvae  of  Thecidium, 
256. 


observations  on  Brachiopoda,  259. 
Lacazella,  231,  245,  246,  247,  255. 
absence  of  setas  in,  249. 
Barretti,    schizolophian    stage    in, 

279. 

blastula  cavity  in,  248. 
difference  in  valves  in,  234. 
hinge  in,  236. 
lophophore  in,  279. 
mediterranea,  lophophore  in,  280. 

schizolophian  stage  in,  279. 
mesembryo  in,  248. 
metembryo  in,  248. 
relation  to  other  brachiopods,  255. 
tentacular  multiplication  in,  Kova- 
levski, 407. 
Lakhmina,  243. 
Lambert  family,  account  of,  Haeckel, 

92. 

lamellibrauchs,  Waldron,  313. 
Lang,  A.,  cephalic  region  in  trilobites, 
165. 

classification  of  trilobites,  114. 
form  of  original  Crustacean,  189. 
nauplian  limbs,  192. 
Laqueus,  231,  245,  301,  306,  308. 
californica,  299. 
connecting  bands  in,  299. 
muhlfeldtiform  stage  in,  301. 
Larnacalpes  lentellipsis,  spines  in,  48. 
larvae,  trilobite,  variations  in,  179. 
larval  stages  of  insects,  spines  in,  19. 
law  of  morphogenesis,  application  of,  1 7. 
of  multiplication  of  effects,  sum- 
mary of  operation  of,  Spencer,  6. 
of  radial  symmetry,  Haeckel,  Jack- 
son, Korshelt,  Heider,  236. 
of  use  and  disuse,  28. 
eaves,  reduction  of,  73. 
Deidy,  Joseph,  freshwater  Khizopoda, 

43. 
Leiolichas,  150. 

glabella  in,  150. 
pygidial  margin  in,  151. 
!eiolophus  stage,  277. 
Leiorhynchus,  246. 

Leiothyrina  cubensis,  loop  in,  Dall,  396. 
Lepas  fascicularis,  defensive   spines  in 
nauplius  larva  of,  Balfour,  56. 
epidurus,  plate-like  telson  in,  84. 
trunk  segments  and  appendages  of) 
191. 


618 


INDEX 


Leptcena,  231,  245,  284,  395. 

absence  of  punctaj  in  deltidium  of, 
260. 
of  straight  hinge-line  in  young, 

268. 

dorsal  fissure  in,  263. 
pedicle-opening  in,  241. 
rhomboidalis,  315. 

and  Strophonella  striata,  paral- 
lel development  in,  333. 
aperture  of  ventral  valve  in, 

394. 

chilidium  in,  264. 
development  of,  325. 
discussion  of,  322. 
geniculated  curtain  in,  397. 
hinge  in,  333. 
neanic,  258, 
nepionic  stage  in,  268. 
striaa  in,  397. 

spiniform  processes  in,  78. 
spiral    impressions    on    valves  of, 

276. 

Leptcenisca,  245,  256. 
Leptobolus,  243,  394. 
Leptocoelia,  245. 

Leptodora,  first  pair  of  appendages  of, 
191. 
trunk  segments  and  appendages  of, 

191. 

Leptoplastus,  142,  145. 
Lernceascus    nematoxys,   suppression  of 

limbs  in,  83,  85. 
lianes,  87. 

Lichadidae,  129,  131,  139,  157. 
defined,  150. 
differentiation  and  specialization  in, 

151. 

lobes  in  glabella  in,  148,  149. 
Lichas,  129,  150,  157. 
glabella  in,  135,  150. 
pygidium  in,  149. 
scabra,  spiniform  processes  in,  78, 

79. 
Lima,  development  of  spines  in,  14. 

squamosus,  spines  in,  51. 
Limnaida,  first  pair  of  appendages  of, 

191. 

limnloids,  aquatic,  suppression  of  ap- 
pendages in,  83. 

classed  with  arachnids,  109. 
tail  spine  in,  84. 


Limulus,  109,  225. 

appendages  in  embryo  of,  Packard, 
209. 

development  of  anterior  legs  in,  84. 

free-cheeks  in,  118. 

optic  nerve  in,  223. 

polyphemus,  spines  in,  22. 
telson  spine  in,  84,  85. 

tail  spine  in,  84. 
Lindstrcemella,  244. 
Lingula,  231,  238,  243,  255,  284, 387,  394. 

adult  brachia  in,  282. 

arms  of,  276. 

complanata,  holoperipheral  mode  of 
growth  in,  266. 

development  of,  Brooks,  256. 

difference  in  valves  in,  234. 

elongate  form  of  shell  in,  287. 

gibbosa.  315. 

inception  of,  Hall  and  Clarke,  286. 

ontogenetic  stages  of,  286. 

outline  and  hinge  in,  238. 

pedicle  in,  235. 

-opening  in,  241. 

protegulum  in,  234. 

pyramidata,  development  of,  388. 

riciniformis,  holoperipheral  mode  of 
growth  in,  266. 

tentacles  in,  275. 

of  taxolophus  in,  277. 
Lingulae,  fossil,    casts    of    pedicles  of, 

Davidson,  Walcott,  257. 
Lingulasma,  243. 
Lingulella,  387. 

Lingulidae,  Obolella,  stage  in,  267. 
Lingulops,  231,  243. 
Linnarssonia,  231,  244. 

protegulum  in,  234. 
Liostracus,  142,  145,  172,  179. 

cephalon  in  protaspis  forms  of,  182. 

early  protaspis  stage  in,  123. 

eye-line  in,  125,  179. 

eyes  in,  126,  180. 

glabella  of  larval  stages  of,  141. 

onangondianus,  171. 
Liothyrina,  231,  246,  247,  291,  307. 

calcareous  loop  in,  281. 

dorsal  pedicle  muscles  in  larva  of, 
251. 

fleshy  portion  of    arms  in,  Han- 
cock, 292. 

loop  in,  292. 


INDEX 


619 


Lithodes,  70. 

maia,  spines  in,  56. 
lizards,  horns  in  male,  Darwin,  59. 

position  of,  in  defence,  54. 
lobster,  development  of  tail  in,  83. 
locust,  common,  spiniform  stipules  of, 
75. 
honey,  compound  thorns  on,  68. 

spiniform  branches  of,  75. 
thorns  of,  10. 
Loganellus,  142,  145. 
Lonchodomus,  138,  220. 
lophophore  in  recent  Brachiopoda,  274. 
Lothelier,  M.,  disappearance  of  spines 

on  barberry,  74. 
Lucanus  cervus,  horns  in,  60. 
dama,  horns  in,  60. 
titanus,  horns  in,  60. 
Lucas,  F.  A.,  change  in  fore  limb  of 

hoactzin,  84. 

Lucifer,  beginnings  of  metastoma  in, 
192. 

nauplius  of,  164. 

Lydekker,  R.,  simple  antlers  of  young 
deer  and  elk,  24. 

antlers  of  red  deer,  1 7. 
Lyopomata,  Owen,  242. 
Lyra,  306,  308. 
Lyttonia,  245. 

M 

Macandrevia,  231,  246,  297,  299,  300, 
301,  306,  308,  411. 
cranium,  297,  300,  409. 

adult  characters  of    loop   in, 

295. 
brachial   supports    in,  Friele, 

Deslongchamps,  406. 
development  of  loop  in,  Friele, 

Deslongchamps,  292. 
disappearance  of  connecting  bands 

and  septum  in,  306. 
ismeniform  stage  of,  300. 
loop  in,  292. 

miihlfeldtiform  stage  in,  301. 
spinose  spires  in,  260. 
stages  of  growth  in,  299. 
McCoy,  F.,  classification  of  trilobites, 

111. 

Machcerium,    stipules     converted    into 
thorns  in,  Schenck,  89. 


Madreporaria    Perforata,   non-tabulate 

protocorallum  for,  427. 
magadiform  stage,  292. 
Magas,  246,  288,  301,  303,  306,  308. 
Magasella,  288,  296,  303,  308. 
Cumingi,  294,  296,  301,  302. 

hinge-plates  and  muscular  ful- 
cra in,  305. 

deltidial  plates  in,  396. 
magaselliform  stage,  292. 
Magellania,  231,  246,  284,  288,  293,  294, 
295,  296,  297,  299,  300,  308. 
cirri  in,  291. 
coiled  arm  in,  276. 
disappearance  of  connecting  bands 

and  septum  in,  306. 
flavescens,  absence  of  deltidial  plates 
in  young,  262. 

adult  characters  of  loop  in,  295. 
beak  of,  263. 

deltidial  plates  in,  262,  263. 
development  of,  294. 
extensions  of  mantle  in,  263. 
gwyniform    stage    in    young, 

294. 

umbonal  portion  of,  262. 
kerguelenensis,   adult  characters  of 
loop  in,  295. 

lophophore  in,  280. 
lenticularis,  adult  characters  of  loop 
in,  295. 

development  of,  294. 
pedicle  in,  235. 

-opening  in,  242. 
plectolophus  stage  in,  284. 
stages  of  growth  in,  299. 
tentacles  in,  275. 

in  young  stages  of,  275. 
of  taxolophus  in,  277. 
trocholophus  stage  in,  278. 
venosa,  297. 

adult  characters  of  loop  in,  295. 
brachial  supports  in,  Fischer 

and  CEhlert,  406. 
development  of  loop  in,  293, 

294. 
(Waldheimia)  septigera,  297. 

well-developed  cardinal  process 

in,  296. 
Wyvillii,  adult  characters   of 

loop  in,  295. 
magellaniform  stage,  292. 


620 


INDEX 


Magellaniinse,  301,  302. 

austral  distribution  of,  304. 
defined,  308. 

development  of,  293,  294. 
median  septum  in,  296. 
stages  of  growth  in,  288,  303. 
Magilus,  attenuation  in,  91. 
Malacostraca,  114,  115,  202. 

affinities  of  trilobites  to,  164. 
Mammalia,  accelerated  development  of 

spines  in,  22. 

mammals,  hollow-horned,  horns  for  pro- 
tection and  offence  in,  54. 
mandibles  of  Triarthrus,  206. 
Mannuopsis  typica,  205. 
Margaritiphora  Jimbriata,  spines  in,  51. 
Marsh,    O.    C.,   antlers   as    sounding- 
boards,  54. 
horns  in  two  sexes  of  Protoceras,  58. 

on  head  of  Triceratops,  54. 
spines  on  tail  of  Stegosaurus,  54. 
variations  in  direction  of  horns,  54. 
Martinia,  231,  246. 
Martinopsis,  246. 

Matthew,  G.  F.,  development  of  trilo- 
bites, 310. 

metamorphoses  of  trilobites,  166. 
protaspis  stage  of  Microdiscus,  136. 
stages  of  trilobites  from  Cam- 
brian rocks  of  New  Bruns- 
wick, 171. 

small  cephala  of  Ctenocepkalus 
(Hartella)  Matthewi  and  Cono- 
coryphe  (Bailielld)  Baileyi,  from 
Cambrian  of  New  Brunswick, 
178. 

maxillae  of  Triarthrus,  206. 
Meelcella,  245. 

Megalanteris,  246,  291,  305,  307. 
Megalaspides,  146. 
Megalaspis,  146. 
Megathyriua?,  302. 
defined,  308. 
development  in,  300. 
Megathyris,   238,   246,    284,    300,    304, 
308. 

decollata,  labial  appendages  of,  279. 
lophophore  in,  275,  279,  280. 
nodes  and  ridges  on,  and  pedicle  in, 

305. 

ptycholophus  stage  in,  284. 
Megerlia,  238. 


Megerlina,  231,  246,  288,  294,  295,  303, 
308. 

beginnings  of  primary  loop  in,  301. 
development  of,  295. 
Jeffreysi,  299. 
Lamarckiana,  295. 
(=Muhlfddtia)  truncata,  299. 
megerliniform  stage,  292. 
Melo  diadema,  spiniform  prominences 

on,  46. 

Menocephalus,  142. 
Merista,  246. 
Meristella,  246. 
Maria,  377. 
rectirostris,  372. 
Meristellidae,  fleshy  portions  of  brachia 

in,  283. 
Meristina,  246,  387. 

absence  of  embryonal  sinus  in,  388. 
Maria,  314,  377,  388,  392. 
beak  in,  388,  389. 
deltidial  plates  in,  390. 
discussion  of,  377. 
triangular  area  in,  390. 
nitida,  374. 
rectirostris,  315,  378. 

absence  of  deltidial  plates  in 

adult,  261. 
beak  in,  389. 
discussion  of,  372. 
embryo  of,  377. 
incipient  stage  in,  376. 
Merostomata,  132,  184. 
culmination  of,  183. 
eyes  of  some,  222. 
included  in  Aspidonia  of  the  Crus- 
tacea, 112. 
mesembryo,  121,  247,  248. 
Mesokaulia,  242. 
Mesonacis,  113,  142,  143,  144. 
asaphoides,  177. 
pygidium  in,  143. 
vermontana,  144. 
Mesothyra,  three  spines  in,  84. 
Mesotreta,  244. 
metagerontic  stage,  248. 
metastoma  in  Triarthrus,  209. 
metembryo,  247,  248. 
Metopias,  150. 

glabella  in,  150. 
Michelinia,  421,  424,  433. 
buds  in,  432. 


INDEX 


621 


convexa,  431. 

corallites  in,  433. 

intermural  cell   multiplication 

in,  430. 
favosa,  spines  on,  39. 

spiniform  processes  on  epitheca 

of,  76,  77. 

lenticularis,  421,  422. 
mural  pores  in,  427. 
size  of  corallites  in,  429. 
Micmacca,  142. 
Microdiscus,  130,  136. 

absence  of  eyes  in,  183. 

cephalon  of,  128. 

eye-spots  lost  in,  135. 

free  segments  in,  136. 

protaspis  stage  of,  Matthew,  136. 

pygidium  in,  135. 

speciosus,  glabella  and  pygidium  in, 

136. 
Milne-Edwards,  A.,  classification  of  tri- 

lobites,  111,  114. 
Mimulus,  245,  387. 
waldronensis,  315. 

discussion  of,  334. 

Minot,  C.  S.,  growth  in  guinea  pigs,  92. 
Moina,  absence  of  median  eye  in,  193. 

nauplius  appendages  of,  192. 
Mojsisovics,  E.,  development  of  fossil 

cephalopods,  310. 
Mollusca,  247. 

attenuation  of  form  in,  91. 
compound  spines  in,  68. 
development  of  spines  in  shells  of, 

13. 

free  variation  among,  65. 
initial  shell  in,  255. 
ontogeny  of  spiniferous  species  of, 

19. 

smooth  early  larval  shells  of,  18. 
protoconch,  periconch,  and  pro- 

dissoconch  of,  18. 
spine  development  in,  14. 
spines  of,  10. 

Jackson,  51. 

spiniform  projections  on  shells  of,  9. 
spiny  forms  of,  70. 
mollusks,  mantle  of,  255. 

mouth  of,  255. 
Monograptus  spinigerus,  apertural  spines 

on,  45. 
Monomer ella,  243. 


Monorachos,  156. 

accessory  lobes  in  glabella  of,  157. 
moose,  antlers  as  sounding-boards  in,  54. 
morphogenesis,  application  of  law  of,  17. 

law  of,  Hyatt,  229. 
Morris,   C.,    defence    in   animals  and 

plants,  52,  97. 

Morse,  E.  S.,  arm  development  in  Tere- 
bratulina,  274. 
comparisons    and    homologies    in 

Brachiopoda,  253. 
crura  in  nepionic  stages  of  Terebrat- 
ulina septentrionalis,  268. 
development  of  Terebratulina,  238. 
early  embryology  of  brachiopods, 
246. 

stages  of  Terebratulina,  231. 
growth  of  loop  in  Terebratulina,  291. 
incipient  stage  of  Terebratulina,  386. 
loop  in  Terebratulina  septentrionalis, 

396. 

tentacular  multiplication  in    Tere- 
bratulina, 407. 
moth,  early  thorn,  mimetic  features  in, 

Poulton,  62. 

motion  primarily  rhythmic,  Spencer,  67. 
Muhlfeldtia,  231,  238,  246,  288,303,306, 
307,  308,  409. 
adult  loop  in,  291. 
attachment  of  loop  in,  292. 
ismeniform  stage  of,  300. 
pedicle  in,  235. 
sanguinea,  298,  300. 

adult  structure  of,  301. 
brachial    supports    in,  Friele, 

Deslongchamps,  406. 
truncata,  298,  299,  300. 
variations  in,  77. 
var.  monstruosa,  production  of 
discinoid  characters  in,  238. 
muhlfeldtiform  stage,  298. 
Miiller,  F.,  description    of    Discinisca, 
256. 

growth  of  spiral  arms  in  Dis- 
cinisca, 282. 

larval  form  of  Discinisca,  246. 
O.  F.,  the  name  Nauplius,  188. 
Murex,  70. 

compound  spines  in  many  species 

of,  68. 

spineless  nepionic  stages  of  spiny, 
18. 


622 


INDEX 


spines  in,  17. 
musk-ox,  horns  in,  53. 
Myriapoda,  214. 
Mr/sis,  nauplius  limbs  in,  192. 

variation  in  anterior  antennas  of, 
192. 


N 


nauplian  limbs,  Lang,  192. 
Nauplius,  the  name,  Miiller,  188. 
nauplius  larva  of  Lepas  fascicularis,  de- 
fensive spines  in,  Balfour,  56. 
crustacean,  188. 
Claus,  190. 

of  modern  higher  Crustacea,  121. 
spineless,  of  Crustacea,  19. 
variation  in,  Balfour,  191. 
Nautiloidea,  radicle  types  in,  95. 
Nautilus  pompilius,  437. 
neanic  period  in  Brachiopoda,  268. 
stage,  248. 

of  trilobites,  121. 
Nebalia,  115. 

nauplius  limbs  in,  192. 
neoembryo,  121,  247. 

development  of,  249. 
Neothyris,  288. 

lenticularis,  301. 
Neotremata,  242,  247,  282. 

defined,  244. 

nepionic    period    in    development    of 
Brachiopoda,  267. 
stage,  248. 

of  trilobites,  121. 
stages  of  Avicula  sterna,  20. 

spineless,  of  spiny  Murex,  18. 
nerve  force,  32. 
Neseuretus,  142. 

Neumayr  and  Paul,  differentiation  of 
Slavonian  Paludina   in    Lower  Pli- 
ocene, 66. 
neurism,  32. 
Newberna,  306,  411. 
Nicholson,  H.  A.,  spinules  on  tubes  of 
Spirorbis,  45. 
and  Lydekker,  R.,  apertural  spines 

on  some  graptolites,  45. 
Nieszkowskia,  155. 

cephalon  and  pygidium  in,  155. 
Nileus,  146. 

eyes  in,  147. 


Niobe,  146. 

smooth  glabella  in,  126. 
Noteus  quadricornis,  spines  on  loricae  of, 

45. 

Nudeatula,  306,  411. 
Nucleospira,  246,  387. 

pisiformis,  315. 

Nucleospiridas,  primary  lamellas  in,  416. 
Nucula,  correlation  of,   with  prodisso- 

conch  of  Pelecypoda,  Jackson,  23. 
nuculoid    radicle    for    Aviculidse    and 

allied  forms,  23. 
numerical  intensity,  Gratacap,  35. 


Obolella,  243,  267,  286. 

cingulata,  232. 

stage,  267. 

in  Lingulidae,  267. 
Obolus,  231,  243,  387,  394. 

labradoricus,  233. 

var.  swantonensis,  233. 

pulcher,  cancellated  nepionic  stage 

in,  267. 
Octillcenus,  146. 

eyes  in,  147. 
Odontocephalus,  156,  157. 

denticulated  cephalon  in,  157. 
Odontopleura,  151,  152. 
(Ehlert,  D.-P.,  dorsal  foramen  in  some 
Brachiopoda,  264. 

families  of  Brachiopoda,  243. 

hypostoma  in  Trinudeus,  135. 
(Ehlertella,  231,  244. 
Ogygia,  145,  146,  154. 

glabella  in,  180. 
Ogygiopsis,  146. 
Oldhamina,  245. 

lophophore  in,  280. 
Olenelloides,  142,  143,  144. 

armatus,  144. 

characters  of,  143. 
Olenellus,  142,  143,  144,  178. 

asaphoides,  young  of,  Ford,  Wai- 
cott,  178. 

facial  sutures  in,  143. 

glabella  in,  180,  181. 

(Mesonacis)  asaphoides,  177. 

pygidium  in,  143. 

retrally  directed  pleura  of,  79. 


INDEX 


623 


tail  spine  in,  84. 

Thompsonl,  144. 
Olenida,  Haeckel,  112. 
Olenidse,  129,  131,  139,  157. 

defined,  141. 

eye-line  in,  141. 
Oleniiiae,  145. 
Olenoides,  142,  145. 
Olenus,  129,  139,  142,  143,  145. 

cephalon  of,  129. 

eye-line  in,  223. 
Oncholichas,  150. 

glabella  in,  150. 
Onomis  korrida,  74. 

splnosa,  spiny  variety  of,  74. 
ontogeny  and  phylogeny  in  Brachiop- 
oda,  correlations  of,  286. 

a  standard  of  classification,  120. 

early  stages  in,  Hyatt,  95. 

of  a  spinose  individual,  18. 

of  spiniferous  species  of  Mollusca, 

19. 

Onycopyge,  153,  155,  156. 
Opisthocomus  cristatus,  thumb  and  index 

finger  in,  84. 

Opisthoparia,   124,  128,   129,  131,  132, 
139,  140,  141,  151,  152,  153. 

defined,  138. 
Orbiculoidea,  231,  244,  268. 

development  of,  239. 

discoid  form  of  shell  in,  287. 

early  stages  of  Paleozoic,  287. 

first  appearance  of,  287. 

minuta,  brachial  valve  of,  233. 

ontogeny  of,  287. 

pedicle-opening  in,  241. 

protegulum  in,  230,  234. 

nepionic  stage  in,  267. 

stages  of  development  in,  265,  266. 
Orbulina  universa,  pelagic  forms  of,  66. 
Orchestidas,  epimeral  and  tergal  spines 

in,  36. 

orders,  foundation  of,  Agassiz,  242. 
Or  easier  gigas,  spines  in,  51. 

occidentalis,  spines  in,  51. 
organisms,  correlations  of  volumes  and 
surface  of,  Ryder,  27. 

parasitic,  life  history  of,  40. 

repetition  of  parts  universal  among, 

Bateson,  67. 

organs,  rudimentary,  origin  of  so-called, 
Darwin,  80. 


origin,  categories  of,  26. 
Ornithopoda,  functional  digits  in,  85. 
Ornithorhynchus,  spur  on  hind  legs  of, 

9,59. 

Orosphcera  Huxleyi,  spines  in,  48. 
Orthidae,  317. 

deltidial  development  in,  393. 
Orthis,  256,  394,  395,  399,  401. 
acutiloba,  400. 
biloba,  399. 
dorsal  fissure  in,  263. 
elegantula,  317. 
group,  231,  245,  399. 
high  hinge-area  in,  266. 
kybrida,  321. 
pedicle-apertures  on  valves  of,  394. 

-opening  in,  241. 
spiniform  processes  in,  78. 
subnodosa,  315. 
teeth  in,  252. 

Orthisina,  245,  252,  257,  395. 
pedicle-opening  in,  241. 
Orthothetes,23l,  245. 

absence  of  straight  hinge-line  in 

young,  268. 

cavity  of  pedicle-sheath  in,  395. 
deltidium  of,  386. 
subplanus,  315. 

aperture  of  ventral  valve  in, 

394. 
concentric   ornamentation    in, 

397. 

deltidial  plates  in,  394,  395. 
development  in,  333. 
discussion  of,  327. 
hinge  in,  333. 
tennis,  315. 

Oryctocephalinse,  145. 
Oryctocephalus,  142,  143,  145. 
Ostracoda,  114,  184. 

development  of  extinct,  Verworn, 

310. 

respiration  in,  218. 

ostraeiform  growth  in  Spondylus,  20,  21. 
Ostrea,  relation  to  Avicula,  255. 
smooth  prodissoconch  of,  18. 
virginiana,  prodissoconch  of,  20. 
Ovibos  moschatus,  horns  in,  53. 
Owen,  K.,  Arthropomata  and  Lyopom- 
ata,  242. 

antlers  in  deer,  22. 
ox,  development  of  horns  in,  14. 


624 


INDEX 


horns  of,  21. 
oyster  and  its  allies,  observations  made 

upon,  Jackson,  237. 
oysters,  spiny,  23. 


Packard,  A.  S.,  appendages  in  embryo 
of  Limulus,  209. 

cause  of  spines  in  Schizurse,  80. 
geological  development   of    Trilo- 
bita,  Brachiopoda,  and    Ammo- 
noidea,  98. 
mimicry  in  caterpillar  of  Schizura, 

62. 
origin  of  spines,  etc.,  in  caterpillars, 

55. 

protective  spines  and  tubercles  in 

caterpillars  of  certain  moths,  65. 

results  of  hypertrophy  in  Crustacea, 

38. 
spines    on    larvae     of    geometrid 

moths,  46. 
Paget,  James,  results  of  pressure  on  an 

organism,  28. 

Palcemon,  embryos  of,  192. 
Palaeocarida,  114. 
PalcBOpyge,  142. 
palm,  climbing,  length    of   steins   in, 

Kerner,  88. 
palms,  spiniferous,  72. 
Paludina   Hcernesi,    differentiation   in, 
66. 

Neumayri,  66. 

Slavonian,    differentiation     of,    in 
Lower   Pliocene,  Neumayr   and 
Paul,  66. 
Panderia,  146,  147. 

position  of  eyes  in,  147. 
Parabolina,  142,  145. 
Parabolinella,  142,  145. 
Paradoxides,  113,  142,    143,    144,   154, 
178. 

Cambrian,  183. 

free  thoracic  segments  in,  183. 
glabella  in,  180. 
large  eyes  in,  183. 
pygidium  in,  143. 

spinosus,  spiniform  pleura!  exten- 
I  sions  of,  79. 
young,  142. 


Paradoxinas,  143. 
paragerontic  stage,  248,  270. 
Parameecia,  231. 

parasitic  organisms,  life  history  of,  40. 
Parodiceras  discoideum,  differences  in, 
436. 

( Goniatites)  discoideum,  436. 
Paterina,  233,  267,  286. 
labradorica,  233. 
new  genus,  232. 
stage,  233,  267,  268. 
the  prototype  of  brachiopods,  286. 
valves  of,  233. 
Paterula,  243. 

pear,  loss  of  spines  of,  by  cultivation, 
74. 

spiniform  branches  of    neglected, 
75. 

termination  of  stems  of,  10. 
Peckham,   E.   G.,   protective    mimetic 

features  in  Madagascar  spider,  61. 
Pecten,  23. 

development    of    spines    in   many 

species  of,  14. 

-like  stage  of  Spondylus,  20,  21. 
Pedicle-opening,  development  of,  Clarke, 
240. 

-openings,  characters  of,  Deslong* 
champs,  240. 
of  Brachiopoda,  38. 
types  of,  240. 
Pelecypoda,  230,  255. 

larval  shell  of,  19,  20. 
phylogeny  of,  Jackson,  23. 
prodissoconch  of,  19,  20,  23,  122. 

Jackson,  231. 
pelecypods,    spines    on    post-umbonal 

slope  of,  45. 
Peltura,  142,  145. 
Peneus,  187,  188. 
eyes  in,  117. 
Pentagonia,  246. 
Pentamerella,  245. 
Pentamerus,  257. 
Peregrinella,  246. 
Perforata,  ancestry  for,  425. 
periconch,  smooth,  of  Mollusca,  18. 
Phacopidae,  129,  131,  152,  153. 
defined,  156. 
schizocroal  eyes  in,  157. 
Phacopini,  152. 
Phacops,  113,  129,  143,  156,  222. 


INDEX 


625 


a  normal  trilobite  type,  1 27. 
plain  cephalon  in,  157. 
pleural  spines  in,  80. 
Phaethonides,  174. 
Phaetonella,  148. 
Pharostoma,  154. 

glabella  in,  154. 
PMiipsia,  143,  148. 
eyes  in,  148. 

fixed-cheeks  in,  127,  148. 
glabella  in,  148. 
Phillipsinella,  146. 
Pholidops,  231,  244. 
ovalis,  315. 

pedicle-opening  in,  241. 
phrenism,  32. 

Phrynosoma,  development  of  horns  in, 
47. 

extreme  spinosity  in,  70. 
mimetic  characters  of,  63. 
prickly  scales  of,  9. 
phylembryo  in  Brachiopoda,  248. 

in  trilobites,  Jackson,  121. 
Phyllocarida,  184. 

culmination  of,  183. 
development  of  anterior  legs  in,  84. 
tail  spine  in,  84. 
three  spines  in,  84. 
Phyllopoda,  114,  184. 
phyllopods,   variation  in  nauplius   of, 

191. 

Phyllopteryx  eques,  mimetic  features  of, 
63. 

protective  characters  of,  55. 
phylogeny  of  Avicula,  Anomia,  Spondy- 
lus,  23. 

of  spinous  forms,  23. 
of  the  Pelecypoda,  Jackson,  23. 
physiogenesis,  6. 

Physospongia  Dawsoni,  spiniform  pro- 
cess in,  50. 
pipe-fish,  70. 

Australian,  mimetic  features  of,  Gun- 
ther,  63. 
protective    characters    of,   Giin- 

ther,  55. 
Fire,  L.,  pathological  varieties  of  recent 

Planorbis  complanatus,  91. 
Placocista  spinosa,   mitre-shaped  form 

of,  with  numerous  spines,  43. 
Placoparia,  129,  153. 
cephalon  in,  154. 


Plcesiocomia,  154. 

glabella  and  pygidium  in,  155. 
Planorbis  complanatus,  pathological  vari- 
eties of  recent,  Pire,  91. 
costatus,  differentiation  toward  spi- 
nosity in,  66. 
pathological  varieties  of  Steinheim, 

Hyatt,  91. 
Steinheim,  evolution  of   Tertiary 

species  of,  Hyatt,  65. 
plants,  climbing,  general  spininess  of, 
88. 

number  of  species  of,  Kerner, 

87. 
suppression  of  structures   in, 

87. 

defence  in,  Morris,  52. 
desert,  atrophy  of  leaves  of,  caused 
by  disuse,  82. 

differentiation  of  parts  of,  into 

spines,  Henslow,  74. 
spines  in,  39,  71. 
parasitic,  spmiform  scales  in,  83. 
primitive  type  of  structure  in,  26. 
spineless  young  seedlings  of,  19. 
spiny  varieties  of,  Henslow,  74. 
thorny,  decline  of  vitality  in,  97. 
xerophilous,    origin    of,    Wallace 

and  Henslow,  74. 

Platidia,  246,  288,  296,  300,  303,  307, 
308. 

adult  loop  in,  291. 
cistelliform  stage  in,  288,  302. 
elimination  of  deltidial  plates  in, 

305. 

paragerontic  tendency  in,  271. 
platidiform  stage  in,  298. 
zugolophus  stage  in,  281. 
Platyceras,  69. 
Platymetopus,  150. 

glabella  in,  150. 
Platypeltis,  146. 
Platystrophia,  245. 

biforata,  404,  405. 
Plectambonites,  245,  256. 

absence  of  straight  hinge-line  in 

young,  268. 
transversalis,  315. 
plectolophus  stage,  280,  284. 
Pleurodictyum,  421,  422. 
Aulopora  stage  in,  427. 
lenticulare,  casts  of,  426. 


40 


626 


INDEX 


development  of,  422. 
initial  cell  in,  426. 
non-tabulate  feature  of,  427. 
normal  and  abnormal  growths 

in,  424. 

simple  cell  growth  and  multi- 
plication in,  429. 
mural  pores  in,  427. 
problematicum,  casts  of,  426. 

initial  pores  in,  426. 
Pleurostomella  alternans,  single  spine  in, 

44. 

plum,  loss  of  spines  of,  by  cultivation, 
74. 

spiniform  branches  of  neglected,  75. 
Plutonides,  142. 
Podocyrtis  Schombwgki,  terminal  spine 

in,  44. 

Polymorphina  Orbignii,  compound  spines 
in,  68. 

sororia,  var.  cuspidata,  single  spine 

in,  44. 

Polyzoa,  252. 
Porambonites,  245. 
porcupine,  extreme  spinosity  in,  70. 
-men,  92. 

modified  hairs  of,  9. 
protection  of,  by  spines,  54. 
Porifera,  mural  pores  in,  76. 
Poulton,  E.  B.,  mimetic  features  in  early 

thorn  moth,  62. 
praemagadiform  stage,  292. 
pressure,  results  of,  on  an  organism, 

Paget,  28. 

Prestwichia,  tail  spine  in,  84. 
prickle,  definition  of,  9. 
prickles  on  climbing  plants,  90. 

on  rose  and  barberry,  9. 
Prionopeltis,  148. 
eyes  in,  148. 
fixed-cheeks  in,  148. 
Pritchard,  A.,  spine  growth  in  Arthro- 

desmus  octocornis,  43. 
Proboscidella,  245. 

difference  in  valves  in,  237. 
Proceratopyge,  142,  145. 
Procervulus,  antlers  in  both  sexes  of,  57. 
processes  on  Echinodermata,  9. 

spinous,  on  the  plant  body,  Bailey, 

90. 

prodissoconch     of     Anomia     acukata, 
young,  20. 


of  Avicula  sterna,  20. 
of  Ostrea  virginiana,  20. 
of  Pelecypoda,  19,  20. 
of  Spondylus,  20. 
smooth,  of  highly  spinose  spe- 
cies of  Spondylus,  18. 
of  Mollusca,  18. 
of     Ostrea,     Anomia, 

Avicula,  18. 

Productella,  69,  231,  245,  270. 
giganteus,  270. 
spine  growth  in,  96. 
Producti,  spiny,  96. 
Productidse,  extinction  of,  96. 
Productus,  69,  237,  245,  260,  270,  395. 
absence  of  straight  hinge-line  in 

young,  268. 
spine  growth  in,  96. 
spiral  impressions  on  valves  of,  276. 
Proetidce,  129,  139,  141. 
defined,  148. 

fourth  annulation  in,  127. 
lobes  in  glabella  in,  149. 
Proetus,  113, 129, 141, 143, 148, 176, 177, 
179. 

decorus,  175. 
eyes  in,  126,  148. 
features  of  dorsal  shield  of,  182. 
fixed-cheeks  in,  148. 
free  cephalic  segments  in,  118. 
glabella  in,  135. 
larvae  of,  128. 
lobation  of  glabella  in,  148. 
parviusculus,  175. 

original    description  of,  Hall, 

175. 

projections,  spiniform,  on  shells  of  Mol- 
lusca, 9. 

prominences,  spinous,  on  test  of  Crusta- 
cea, 9. 

Proparia,  124,  129,  131,  132,  153. 
defined,  152. 
earliest  forms  of,  152. 
history  of,  31. 
Prosopiscus,  153. 
Protagraulus,  142. 
Protaspis,  167,  169. 
characters  of,  169. 
geological  history  of,  123. 
homologies  of,  170. 
restoration  of,  185. 
stage  in  trilobites,  121,  122. 


INDEX 


627 


protegulnm,  230. 

affinities  of,  232. 

modifications  in,  from  acceleration, 
233. 

smooth,  of  Brachiopoda,  18. 
protembryo,  120,  247,  248. 
Proteus,  external  limbs  in,  76. 
Protoceras,  horns  in  two  sexes  of,  Marsh, 

58. 
protoconch,  plain,  of  cephalopod,  18. 

smooth,  of  Mollusca,  18. 
Protolenus,  142. 
Protopeltura,  142,  145. 
Protoplasta,  43. 
Protremata,  242,  247,  282. 

decline  in,  96. 

defined,  244. 

deltidium  in,  257. 

life  history  of,  Schuchert,  95. 

punctate  genera  of,  260. 
Protrilobita,  Haeckel,  112. 
Protypus,  142,  145. 
Pseudocrania,  244. 
Pseudophillipsia,  148. 
Pseudosphcerexochus,  155,  156. 
Psilocephalus,  146. 

position  of  eyes  in,  147. 
Pterinopecten,  23. 

spondylus,  spines  in,  51. 
Pterocephalia,  142. 
Pterophloios,  284. 

lophophore  in,  280,  281. 

ptycholophus  stage  in,  284. 
Pterygometopus,  156. 
Pterygotus,  plate-like  telson  in,  84. 
Ptychaspis,  142,  145. 
ptycholophus  stage,  280,  284. 
Ptychometopus,  154. 

glabella  in,  154. 

Ptychoparia,  129, 139, 140, 142, 143, 145, 
172,  176,  179,  180. 

cephalon  in,  129. 

protaspis  forms  of,  182. 

early  protaspis  stage  in,  123. 

eye-line    in,    125,    126,    179,    180, 
223. 

eyes  in,  126,  180. 

first  protaspis  stage  in,  139. 

free-cheeks  in,  123,  124,  181. 

glabella  in,  180,  182. 

in  larval,  126,  141. 
Kingi,  171,  172. 


Linnarssoni,  171. 

young  specimen  of,  Callaway, 

178. 

ontogeny  of,  135. 
protaspis  stage  in,  136. 
Ptychopyge,  146. 
Pygidiata,  Haeckel,  1 12. 
Pygope,  307. 

python,  probable  origin  of  vestigial  hind 
legs  of,  76. 

reduction  of  limbs  in,  caused  by 

disuse,  82. 
spurs  on,  10,  39. 


Quenstedt,  F.  A.,  classification  of  trilo- 
bites,  111. 


R 


radicle,  nuculoid,  for    Aviculidse  and 

allied  forms,  23. 
Radiolaria,  compound  spines  in,  68. 

Nasellarian,  terminal  spine  on,  43. 

projecting  rays  and  processes  of,  9. 

spine  differentiation  in,  16,  69. 

spine  production  in,  13. 

spines  in,  48. 

Spumellarian,  spines  in,  44. 
Rangifer  tarandus,  antlers  in,  53. 
Raphiophorus,  138,  220. 
ratan,  leaf  of,  89. 

length  of  stems  in,  Kerner,  88. 
rays  and  processes  of  Radiolaria,  9. 
Reed,  F.  C.,  direct  line  from  Cyrtome- 

topus,  156. 

reindeer,  antlers  in,  53. 
Remopleurides,  142,  143,  144. 

pygidium  in,  143. 
Rensselceria,  246,  291,  305,  306,  411. 

loop  in,  281. 

punctate  shells  in,  411. 
reproductive  divergence,  Vernon,  37. 
reptiles,    dinosaurian,     great     horned 
forms  of,  96. 

spiniferous  lines  and  ridges  on,  52. 
restraint,  external,  31,  38,  39. 

of  environment,  31. 
Retaster  cribrosus,  spines  in,  51. 


628 


INDEX 


Reticularia,  246. 

bicostata,  var.  petila,  315. 
development  of,  381. 
discussion  of,  380. 
Retiograptus      tentaculatus,      apertura] 

spines  on,  45. 
Retzia,  246,  366,  395. 

evax,  360. 

Rhinoceros,  spine  production  in,  13. 
Rhipidomella,  245,  387. 

absence  of  embryonal  sinus  in,  388. 
hybrida,  315,  317,  318,  352. 
discussion  of,  321. 
earliest  growth-stages  in,  373. 
mature  individuals  in,  373. 
Rhizopoda,  freshwater,  Leidy,  43. 
Rhombopteria,  23. 

Rhopalastrum  hexaceros,  spines  in,  44. 
hexagonum,  44. 
malleus,  44. 
triceros,  spines  in,  44. 
Rhynchonella,  231, 246, 256, 257, 284, 392, 
395. 

acinus,  339. 
arms  of,  276. 
arm  structure  in,  276. 
brachia  in,  279. 
delthyrium  of  young,  262. 
deltidial  plates  in,  386,  391,  395. 
development  of,  383,  385. 
dorsal  beak  in,  264. 
indianensis,  346. 
neglecta,  341. 
pedicle-opening  in,  242. 
spirolophus  stage  in,  284. 
tentacles  in,  275. 
Whitii,  344,  366. 

Rhynchonellae,  some  Mesozoic,  deltidial 
plates  in,  391. 

umbo  in  ventral  valve  of,  394. 
Rhynchonellidae,  271. 

deltidial  plates  in,  266. 
deltidium  in,  393. 
sheU  in,  234. 

some  Mesozoic,  processes  on  del- 
tidial plates  of,  390. 
Rhynchonellina,  246. 
Rhynchotrema,  246. 
Rhynchotreta,  246,  387. 

cuneata,  315,  350,  352,  358. 

circular  apical  perforation  in, 
389. 


deltidial  plates  in,  390. 

plications  in,  397. 

profiles  of  beaks  in,  389. 

sinus  and  fold  in,  388. 

var.  americana,  discussion  of, 

351. 

Rhynchoporina,  246. 
Rhynobolus,  243. 
rhythm,  Spencer,  28. 
Robinia  Pseudacacia,  spiniform  stipules 

of,  75. 

Rcemerella,  244. 

Romanes,  G.  J.,  spines  caused  by  sup- 
pression of  an  organ,  39. 
Romingeria,  433. 

corallum  in,  433. 
erect  growth  in,  427. 
growth  in,  425. 
periods  of  gemmation  in,  428. 
rose,  loss  of  spines  of,  by  cultivation,  74. 

prickles  on,  9. 

Rotatoria,  spines  on  loricae  of  some,  45. 
Rubus  sguarrosus,  89. 
Rudistes,  236. 
Ruminata,  horns  in,  58. 
Ryder,  J.  A.,  correlations  of  volumes 
and  surface  of  organisms,  27. 


S 

Sabella,  254. 

Sacculina,  absence  of  intestine  in,  193. 
Salter,  J.  W.,  classification  of  families 
of  trilobites,  110,  112,  132. 

metamorphoses  of  trilobites,  166. 
Salteria,  138,  140,  220. 
Sao,  142,  145,  173,  176,  179,  182. 
eye-line  in,  125,  126,  179,  180. 
eyes  in,  126,  180. 
first  protaspis  stage  in,  139. 
free-cheeks  in,  123,  181. 
glabella  in,  180. 

in  larval,  126,  141. 
hirsuta,  172,  176. 

Barrande,  166. 
nepionic,  139,  140. 
ontogeny  of,  128,  135. 
pleura  from  pygidium  in,  127. 
protaspis  stage  in,  136. 
pygidium  in,  181. 
Sar copies  scabiei,  suppression  of  limbs 
in,  83. 


INDEX 


629 


Saxicava  arctica,  spines  on  post-umbonal 
slope  of,  45. 
young,  20. 

scales,  prickly,  of  horned  toad,  9. 
Scenidium,  238,  256,  395. 
Schenck,  H.,  stipules    functioning  as 

thorns  in  Machcerium,  89. 
Schizambon,  231,  244. 

typicalis,  spinules  in,  51. 
Schizobolus,  231,  244. 
Schizocrania,  231,  244. 

pedicle-opening  in,  241. 
stages  of  development  in,  265,  266. 
schizolophus  stage,  278. 
Schizophoria,  245. 
Schizopoda,  200. 
Schizotreta,  231. 

pedicle-opening  in,  241. 
similar  form  in  both  valves,  236. 
Schizura,   caterpillar    of,   mimicry    in, 

Packard,  62. 

Schizurae,  cause  of  spines  in,  Packard,  80. 
Schmidtella,  148. 
Schmidtia,  142,  143,  144. 
Mickwitzi,  144. 
pygidium  in,  143. 

Schuchert,  C.,  brachidium  in  Zygospira, 
282. 

evolution  of  Brachiopoda,  30. 
life  history  of  Protremata,  95. 
rudimentary  teeth  inKutorgina,  252. 
specific     differentiation     in    Stro- 

phomenacea,  96. 
Scutigera,  214. 
Scytalocrinns  validus,  spines  on  terminal 

sacs  of,  45. 

sea-lilies,  smooth  surface  in  early,  25. 
-urchins,   minute    spines    in  early 

genera  of,  25. 
Seeley,  H.  G.,  spines  in  male  Callichthys, 

59. 
selection,  cannibalistic,  Verrill,  57. 

sexual,    as    affecting    growth    of 
antlers  in  deer,  Darwin,  58. 

spines  from,  57. 
Sdenopeltis,  U3,  151,  152. 
Sethastylus  dentatus,  spines  in,  48. 
shell,  larval,  of  Pelecypoda,  19,  20. 

pear-shaped,  of  Difflugia  pyriformis, 

43. 

spherical,  of  Difflugia  globulosa,  43. 
shells,  discoid,  spines  in,  48. 


of  Brachiopoda  and  Mollusca,  de- 
velopment of  spines  in,  13. 
of  Mollusca,  spiniform  projections 

on,  9. 
Shipley,  A.   E.,  early  embryology  of 

brachiopods,  246. 
Shumardia,  133. 
Sieberella,  245. 
Simpson,  C.  T.,  anchoring   spines    on 

Unio  spinosus,  46. 
'Siphonotreta,  69,  70,  244. 

unguiculata,  spinules  in,  51. 
Smith,  S.  I,  characters  in  the  append- 
ages of  Trinucleus,  225. 
Solenopleura,  142,  145,  172,  179. 

cephalon    in    protaspis    forms    of, 

182. 

early  protaspis  stage  in,  123. 
eye-line  in,  125,  179. 
eyes  in,  126,  180. 
glabella  of  larval  stages  of,  141. 
Robbi,  171. 

Spencer,     H.,    all    motion     primarily 
rhythmic,  67. 
rhythm,  28. 

summary  of  the  operation  of  the  law 
of  multiplication  of  effects,  6,  7. 
Sphcerexochus,  153,  155. 

fourth  annulation  in,  127. 
Sphserocoryphe,  155,  156. 
Sphceropthalmus,  142,  145. 
spider,  Madagascar,   mimetic  features 

in,  Peckham,  61. 
spiders,  mimetic  forms  of,  61.       .,« 

spiny,  protective  mimetic  features 

in,  62. 
spine,  definition  of,  9. 

development,  evidence  of  old  age 
in,  17. 

in  Brachiopoda,  14. 
in  conical  non-coiled  Gastrop- 
oda, 14. 

in  Mollusca,  14. 
in  Spondylus  imperialis,  12. 
differentiation    in    Radiolaria,    16, 

69. 

direct  and  progressive  growth  of,  10. 
from  frustule    of  Attheya   decora, 
43. 

of  Staurastrum  cuspidatum,  43. 

of  Xanthidium    armatum    and 

Anthrodesmus  octocornis,  43. 


630 


INDEX 


genesis,  summary  of  causes  of,  41. 
growth  in  desmids  and  diatoms,  42, 
43. 
in  Echinoidea  and  Asteroidea, 

69. 

in  Spondylus  calcifer,  17. 
of  a,  10. 
stages  of,  19. 
indirect  and  regressive  growth  of, 

10. 

production  in  Articulata,  13. 
in  cattle,  13. 
in  Crustacea,  13. 
in  deer,  13. 
in  Echinoidea,  13. 
in  elk,  13. 
in  giraffe,  13. 
in  Radiolaria,  13. 
in  Rhinoceros,  13. 

terminal,  in  Eucyrtidium  elegans, 
Podocyrtis  Schomburgki,  Tridic- 
tyopus  conicus,  Cornutella  hexa- 
gona,  44. 

on  Euglypha  mucronata,  43. 
on  Nasellarian  Radiolaria,  43. 
spines,  absence  of,  in  old  age  of  Spondy- 
lus calcifer,  21. 

accelerated  development  of,  in  Acid- 
aspis,  22. 

in  Arthropoda,  22. 
in  giraffe,  22. 
in  Mammalia,  22. 
in  Trilobita,  22. 
barbed,    etc.,    on    thicket    plants, 

Kerner,  88. 
compound,  16. 

in  Asteroidea  and  Crinoidea,68. 
in  Echinoidea,  68. 
in  fishes,  68. 
in  Kadiolaria,  68. 
development  of,  in  Gastropoda,  14. 
in  Lima,  14. 

in  shells  of  Brachiopoda,  13, 
14. 

of  Mollusca,  13. 
in  Spondylus,  14. 
epimeral  and  tergal,  in  Orchestidae, 

36. 

growth  and  differentiation  of  orna- 
ment into,  11. 
minute,  in  early  genera  of  Echi- 
noidea (sea-urchins),  25. 


in  desert  plants,  39. 

indirect  growth   of,    in  barberry, 

12. 

in  Foraminifera,  Brady,  49. 
in  larval  stages  of  Crustacea,  19. 

of  insects,  19. 
in  Murex,  17. 
in  Spumellarian  Radiolaria,  Haec- 

kel,  44. 

of  Brachiopoda,  10. 
of  Crustacea,  10. 
of  Mollusca,  10. 
of  vertebrae,  9. 
on  fishes,  9. 
on  fundus  of  Difflugia  acuminata, 

I),  constricta,  D.  corona,  43. 
on  larvse  of  geometrid  moths,  Pack- 
ard, 46. 

on  Limulus  polyphemus,  22. 
on  Michelinia  favosa,  39. 
on  Placocista  spinosa,  43. 
on  stems  of  pear,  10. 
on  zoea  of  Decapoda,  46. 
origin  and  significance  of,  3. 
passage  from  simple  to  compound, 

17. 

primary  series  of,  in  Spondylus,  17. 
taxonomic  value  of,  Harris,  101. 
what  they  are,  4. 
what  they  represent,  4. 
spinose,  individual,  ontogeny  of,  18. 
spinous  forms,  phylogeny  of,  23. 

growth  in  Spondylus  calcifer,  21. 
processes  on  the  plant  body,  Bailey, 

90. 

spiny  oysters,  23. 

Spirifer,  231,  246,  256,  260,  284,  318, 
387,  395,  399. 

absence  of  embryonal  sinus  in,  388. 
crispa,  380. 

var.  simplex,  380. 
crispus,  315,  352. 

areal  development  of,  390. 
concentric    ornamentation    in, 

397. 
development  of,  381. 

of  deltidial  plates  in,  392. 
discussion  of,  380. 
var.  simplex,  315,  352. 
development  of,  381. 
discussion  of,  380. 
cumberlandice,  383,  385. 


INDEX 


631 


triangular  deltidial  plates  in, 

392. 

development  in,  383. 
dorsal  fissure  in,  263. 
eudora,  315. 

fimbriatus,  spinules  in,  51. 
high  hinge-area  in,  266. 
hirtus,  compound  spines  in,  Hall 

and  Clarke,  68. 
loop  in,  292. 
macronotus,  385. 
medialis,  385. 
muci-onatus,  spiniform  processes  in, 

78. 
niagarensis,  383,  385. 

deltidial  plates  in,  390. 
pedicle-opening  in,  242. 
perlamellosus,  383,  385. 

triangular  deltidial  plates  in, 

392. 

pseudo-deltidium  of,  385. 
pseudolineatns,  spinules  in,  51. 
radiatus,  315,  383,  385. 
beak  in,  389. 
development  of  deltidial  plates 

in,  392. 

discussion  of,  382. 
setigerus,  spinules  in,  51. 
spiniform  processes  in,  77. 
spirolophus  stage  in,  284. 
sulcatus,  383,  385. 
teeth  in,  252. 

Spin/era  licostata  ?  var.  petila,  380. 
radiata,  382. 
?  waldronensis,  334. 
Spiriferacea,  283. 
arms  of,  276. 
brachidium  in,  274. 
Spiriferidae,  283,  392. 

deltidial  plates  in,  266. 
deltidium  in,  393. 
primary  lamellae  in,  416. 
Spiriferina,  246,  383,  392. 

pinguis, deltidial  development  in,393. 
triangular  deltidial  plates  in, 

392. 

pseudo-deltidium  in,  392. 
rostrata,  deltidial  development  in, 
393. 
triangular  deltidial  plates  in, 

392. 
spinose  spires  in,  260. 


Walcotti,  deltidial  development  in, 
393. 

triangular  deltidial  plates  in, 

392. 

Spirigerella,  246. 
spirolophus  stage,  282,  284. 
Spirorbis,  254. 

development  of,  Fewkes,  253. 
spinuli ferns,  spinules  on  tubes  of, 

Nicholson,  45. 
spondylium,  257. 
Spondylus,  70. 

ccdcifer,  absence  of  spines  in  old 
age  of,  21. 

spine  growth  in,  17,  21. 
compound  spines  in  many  species 

of,  68. 

development  of  spines  in,  14. 
imperialis,  process  of  spine  develop- 
ment in,  12. 
oldest  species  of,  23. 
ostraeiform  growth  in,  20,  21. 
Pecten-like  stage  of,  20,  21. 
phylogeny  of,  23. 
primary  series  of  spines  in,  17. 
princeps,  radiating  ridge  of,  10,  11. 
prodissoconch  of,  20. 

smooth,  of  highly  spinose  species 

of,  18. 

sponges,  hexactinellid,  progressive  dif- 
ferentiation of  ornament  in,  Hall,  49. 

Waldron,  313. 
spur,  definition  of,  9. 

on  hind  limbs  of  Echidna  and  Orni- 

thorhynchus,  9,  59. 
spurs  of  Python,  10,  39. 

on  birds,  9,  58. 
star-fishes,  protective  spines  in,  56. 

smooth  surface  in  early,  25. 
Staurocephalus,  155. 
Staurastrum  cuspidatum,  spine  growth 

from  frustule  of,  43. 
Stegosaurus,  spines  on  tail  of,  Marsh,  54. 
stimuli,  external,  from  environment,  31, 

32,  34. 
Stomatopoda,  movable  ocular  segment 

of,  119. 
Streptis,  245. 
Streptorhynchus,  231,  245. 

subplanum,  327. 
Striatopora,  424. 

mural  pores  in,  427. 


632 


INDEX 


Stricklandinia,  245. 

dorsal  fissure  in,  263. 
Stringocephalinae,  291. 

defined,  307. 

Stringocephalus,  246,  291, 305,  307. 
Strombus  pugilis,  spiniform  prominences 

on,  46. 

Strophalosia,  69,  70,  96,  239,  245,  260, 
395. 

hinge  in,  236. 

-line  in  neanic  stages  of,  237. 
keoJcuk,  spines  in,  69. 
succession  of  species  in,  236. 
Stropheodonta,  231,  245. 

absence  of  straight  hinge-line  in 

young,  268. 

cavity  of  pedicle-sheath  in,  395. 
deltidial  plates  in,  395. 
deltidium  of,  386. 
dorsal  beak  in,  264. 
pedicle-opening  in,  241. 
profunda,  315. 
spiniform  processes  in,  78. 
striata,  330. 
Strophomena,  231,  245,  252,  395. 

absence  of  punctas  in  deltidium  of, 
260. 
of  straight  hinge-line  in  young, 

268. 

deltidium  of,  386. 
dorsal  beak  in,  264. 
Paterina-like  stage  in,  268. 
pedicle-opening  in,  241. 
rhomboidalis,  322. 
teeth  in,  252. 
Strophomenacea,  specific  differentiation 

in,  Schuchert,  96. 
Strophomenidae,  387,  394. 

absence  of  embryonal  sinus  in,  388. 
cardinal  area  of,  390. 
deltidial  development  in,  393. 
hinge,  fissure,  and  callosity  in,  333. 
Strophonella,  245,  387. 
semifasciata,  315. 
striata,  315,  327. 

discussion  of,  330. 
groove  in,  333. 
hinge  in,  333. 
Stygina,  146. 

fixed-cheeks  in,  127. 
Stylonurus,  tail  spine  in,  84. 
Suessia,  246. 


symmetry,  radial,  application  of  law  of, 
to  Brachiopoda,  236. 

law    of,    Haeckel,     Jackson, 

Korshelt,  Heider,  236. 
Symphoria,  156. 

accessory  lobes  in  glabella  of,  157. 
Symphysurus,  146. 
Syringopora,  425,  426. 

erect  growth  in,  427. 
Syringothyris,  246. 

high  hinge-area  in,  266. 


tachy  genesis,  128. 

taxolophus  stage,  277. 

Technocrinus  spinulosus,  spines  in,  50. 

Telephus,  142. 

Telotremata,  242,  247,  260,  282. 

defined,  245. 
tendrils,  88. 
Terataspis,  70,  150. 

extremes  of  spinosity  in,  56. 
glabella  in,  150. 

Terebratalia,  288,  292,  299, 301, 303,  306, 
308. 

adult  loop  in,  291. 
cistelliform  stage  in,  408. 
coreanica,  adult  characters  of  loop 
in,  295. 

condition  in,  299. 

frontalis,  adult  characters  of  loop 
in,  295. 

condition  in,  299. 
gwyniform  stage  in,  408. 
Marice,  299. 
new  genus,  297. 
obftoleta,  297,  407,  409. 

adult  condition  in,  299. 
development  of,  298,  406. 
lophophore  in,  280. 
metamorphoses  of,  407. 
septum  in  platidiform  stage  of, 

409. 
spitzbergensis,  299. 

adult  characters  of  loop  in,  295. 
stage  in  Macandrevia  and  Dallina, 

299. 
transversa,  297. 

adult  characters  of  loop  in,  295. 
condition  in,  299. 


INDEX 


633 


tentacles  in,  275. 
trocholophus  stage  in,  278. 
Terebratella,  231,  238,  246,  294,  295, 296, 
297,  299,  303,  308. 
attachment  of  loop  in,  292. 
bouchardiform  stage  of,  301. 
Buckmani,  301. 
chiliensis,  293. 
cruenta,  301. 

adult  characters  of  loop  in,  295. 

development  of,  294. 
deltidial  plates  in,  391. 
descending  lamellae  in,  292. 
development  of,  293. 
dorsata,  292,  297,  301,  407. 

adult  characters  of  loop  in,  295. 

brachial  supports  in,  Fischer 
and  (Ehlert,  406. 

development  of  loop  in,  Fischer 
and  CEhlert,  293. 

different  stages  of,  295. 
furcata,  300. 
obsoleta,  407. 
occidentalis,  407. 

var.  obsoleta,  297,  406. 
rubicunda,  301. 

adult  characters  of  loop  in,  295. 

development  of,  294. 
transversa,  gerontic  period  in,  269. 
Terebratellidse,  271,  279,  283,  292,  293, 
296,  297,  299,  301,  406,  412. 
boreal  stock  of,  306. 
brachial  supports  in,  410. 
brachidium  in,  281. 
cistelliform  stage  in,  278. 
defined,  307. 
development  of,  299. 
gwyniform  stage  in,  407. 
King,  292. 
loop  in,  291. 
lophophore  in,  280. 
metamorphoses  in,  287. 
period  of  reproduction  in,  289. 
septum  in,  306. 
specific  characters  in,  289. 
stages  of  growth  in,  291,  302. 
terebratelliform  stage,  292. 
Terebratula,   246,   257,    291,   392,   395, 
399. 

deltidial  plates  in,  386,  395. 
development  in,  383,  385. 
septigera,  291,  297. 


transversa,  291. 

Whitfieldella,  deltidial  plates  in,  391. 
Terebratulae,  411. 

umbo  in  ventral  valve  of,  394. 
Terebratulacea,  brachidium  in,  274. 

zugolophus  stage  in,  281. 
Terebratulidse,  283,  290,  291,  292. 
brachial  supports  in,  410. 
centronelloid  loop  in,  413. 
coiled  arm  in,  276. 
defined,  306. 
deltidial  plates  in,  266. 
deltidium  in,  393. 
extinct  genera  of,  305. 
loop  in,  290. 
lophophore  in,  280. 

Terebratulina,  231,  238,  246,  247,  284, 
291,  293,  387. 
arm  development  in,  Morse,  274. 

structure  of,  276. 
attachment  of,  256. 
blastula  cavity  in,  248. 
brachia  in,  279. 

bristles  on  cephalic  segment  of,  249. 
calcareous  loop  in,  281,  282. 
cancellata,  adult,  280. 
caput-serpentis,  var.   septentrionalis, 

296. 

cirri  in,  291. 
coiled  arm  in,  276. 
development  of,  Morse,  238. 
difference  in  valves  in,  234. 
dorsal  pedicle  muscles  in  larva  of, 

251. 
fleshy  portion  of  arms  in,  Hancock, 

292. 

growth  of  loop  in,  Morse,  291. 
incipient  stage  of,  Morse,  386. 
larval  features  in,  291 
loop  in,  292,  293. 
mesembryo  in,  248. 
metembryo  in,  248. 
outline  and  hinge  in,  238. 
pedicle  in,  235. 

-opening  in,  242. 
plectolophus  stage  in,  284. 
schizolophus  stage  in,  278. 
septentrionalis,  absence  of  deltidial 

plates  in  young,  262. 
beak  of,  263. 

crura  in  nepionic    stages  of, 
Morse,  268. 


634 


INDEX 


early  stages  in,  280. 
loop  in,  Morse,  396. 
nepionic  stages  of,  268. 
umbonal  portion  of  adult,  262. 
zugolophus  stage  in,  284. 
teeth  in,  252. 
tentacles  in  young  stages  of,  275. 

of  taxolophus  in,  277. 
tentacular  multiplication  in,  Morse, 

407. 

trocholophus  stage  in,  278. 
typembryo  of,  250,  251. 
Terebratulinse,  291,  305,  307. 

defined,  307. 
Terebratuloidea,  246. 
terebratuloids,  septal  and  dental  plates 

in,  Waagen,  297. 
Terebrirostris,  umbo  in,  394. 
test  of  Crustacea,  spinous  prominences 

on,  9. 
Tetracorolla,  spinules  in,  Edwards  and 

Haime,  50. 

Textularia  carinata,  spines  in,  49. 
folium,  pairs  of  spines  in,  44. 
Textularia},  spines  in,  44. 
Thakops,  146. 

position  of  eyes  in,  147. 
Thecidea  (Thecidium),  231,  239, 245,  255, 
257,  259. 
absence  of  punctae  in  deltidium  of, 

260. 

affinities  with  strophomenoids,  256. 
arm  development  in,  Kovalevski, 

274. 

brachia  in,  279. 
cephalula  of,  258. 
characters  of,  Dall,  256. 
difference  of  valves  in,  234, 235, 236. 
early  sedentary  larvae  of,  Kovalev- 
ski, Lacaze-Duthiers,  256. 
high  hinge-area  in,  266. 
hinge  in,  236. 
(Lacazella)  mediterranea,  258. 

lophophore  in,  279. 
lophophore  in,  279,  280. 
ptycholophus  type  of,  284. 
radiata,  lophophore  in,  280. 
researches  on,  Kovalevski,  257. 
tentacles  in,  275. 

of  taxolophus  in,  277. 
trocholophus  stage  in,  278. 
typembryo  of,  258. 


ventral  valve  in,  Kovalevski,  259. 
vermicularis,  lophophore  in,  280. 
Thecidella,  245. 

schizolophus  stage  in,  279. 
Thecidiidae,  279. 

lophophore  in,  276. 
Thecidopsis,  245. 
Thecospira,  246,  270. 
thistles,  spiniform  bracts  in,  75. 
Thoracostraca,  last  pair  of  pleopods  in, 

217. 

thorn,  definition  of,  9. 
thorns,    compound,    on    honey-locust, 
68. 

of  locust  and  barberry,  10. 
thought  force,  32. 

toad,  horned,  development  of  horns  in, 
47. 

extreme  spinosity  in,  70. 
mimetic  characters  of,  63. 
prickly  scales  of,  9. 
toads  and  frogs,  ossification  of  superior 

cranial  walls  in,  Cope,  47. 
Tornoceras,  436,  437. 

development  of  shell  in,  435. 
embryonal  characters  of,  435. 
(Goniatites)  uniangulare,  435. 
protoconch  of,  437. 
retrorsum,   differences  in    develop- 
ment of,  435. 

study  of,  von  Buch,  435. 
var:  typum,  439. 

study  of,  Branco,  435. 
uniangulare,  436. 

development  of,  440. 
growth  in,  435. 
Torosaurus,  96. 
Toxoceras,  96. 
Toxotis,  140. 

glabella  in,  140. 
Trematis,  231,  244. 
Trematopora  echinata,  spinules  of,  45. 

spiculata,  spinules  of,  45. 
Trematospira,  246. 
Triarthrella,  142. 
Triarthrida,  Haeckel,  112. 
Triarthrus,  116,  142,  145,  174,  179,  182, 
204,  207,  208,  210,  214,  215,  221. 
absence  of  eye-line  in  adult,  179, 

180. 

anal  opening  in,  209. 
antennal  organs  in,  211. 


INDEX 


635 


anterior  antennae,  or  antennules  in, 

205. 
Becki,  193,  201,  221. 

additional  thoracic    segments 

in,  Walcott,  217. 
mode  of  occurrence  and  struc- 
ture and  development  of,  1 97. 
endopodites  of,  224. 
eye-line  in,  125,  126,  179. 
eyes  in,  126,  180. 
facial  sutures  in,  125. 
fringes  on  exopodites  of,  218. 
full  number  of  segments  in,  218. 
further    observations    on    ventral 

structure  of,  203. 
glabella  in  larval,  126,  180. 
hypostoma  of,  208. 
legs  of,  199. 
mandibles  of,  206. 
maxillae  of,  206. 
metastoma  in,  209. 
morphology  of,  213. 
mouth,  in,  209. 
ontogeny  of,  135. 
organs  in  the  median  line  of,  208. 
paired  biramose  appendages  in,  205. 
uniramose  appendages  in,  205. 
posterior  antennse  in,  206. 
primitive  characters  of,  216. 
protaspis  in,  186. 
respiration  in,  218. 
thoracic  legs  of,  207. 
ventral  structure  of,  185. 
Triceratops,  96. 

horns  on  head  of,  Marsh,  54. 
Tridictyopus  conicus,  terminal  spine  in, 

44. 

Trigonactura  triacantha,  spines  in,  44. 
Trigonosemus,  246,  301,  306,  308. 
Trilobita,  J12,  114,  115. 

accelerated  development  of  spines 

in,  22. 

a  primitive  type,  202.  • 
classed  with  Aspidonia  of  the  Crus- 
tacea, 112. 
evolution  of,  31. 

geological  development  of,Packard, 
98. 

distribution  of,  133. 
phyllopod    affinities    of,    Bernard, 

210. 
pleural  spines  in,  80. 


protective  spines  on,  56. 

pygidium  of,  78. 

restraint  of  environment  in,  39. 

spineless  protaspis  of,  19. 

spiniform  structures  in,  77. 

spiny  forms  of,  70. 

sub-class,  130,  131. 

tail  spine  in,  84. 

the  earliest   forms   of    Crustacea, 

Woodward,  184. 
trilobite  eggs,  169. 

head,  composition  of,  Kingsley,  188. 
larvae,  variations  in,  179. 
trilobites,  214. 

affinities    of,    Bernard,    Kingsley, 
Woodward,  110. 
to  Entomostraca  and  Malacos- 

traca,  164. 
an  appendage  to    the  Crustacea, 

Lang,  114. 
antiquity  of,  183. 

application  of  principles  for  ar- 
rangement of  families  and  genera 
of,  125. 

arrangement  of  families  of,  131. 
blind,  153. 
characters  of,  116. 
classed  with  arachnids,  109. 

with  Crustacea,  109. 
classification  of,  Barrande,  110. 
Brongniart,  110,  111. 
Burmeister,  111,  112. 
Chapman,  113. 
Corda,  111. 
Dalman,  111. 
Emmrich,  111,  112. 
Gegenbaur,  114. 
Goldfuss,  111. 
Lang,  114. 
McCoy,  111. 

Milne-Edwards,  111,  114. 
Quenstedt,  111. 
Salter,  110,  112,  132. 
Walcott,  114. 

cranidium  in,  Bernard,  181. 
description  of  elementary  forms  of, 

176. 
development  of,  Barrande,  Ford, 

Matthew,  Walcott,  310. 
discussions  of  order  and  families  of, 

134. 
dorsal  shield  of,  Lang,  193. 


636 


INDEX 


free-cheeks  in,  117. 
geological  history  of,  120. 
historical  review  of,  Zittel,  111. 
hypostoma  in,  Bernard,  188. 
larvae  of,  169. 

larval  stages  of,  122,  164,  166. 
metamorphoses  of,  Barrande,  166, 
193. 
Callaway,  Ford,  Matthew,  Sal- 

ter,  Walcott,  166. 
natural  classification  of,  109. 
orders  of  development  in,  Barrande, 

167. 

phylembryo  in,  Jackson,  121. 
protaspis  in,  122. 
stage  of,  121. 

stages  of,  from  Cambrian  rocks 
of  New  Brunswick,  Matthew, 
171. 

rank  of,  114. 

structure  and  development  of,  109. 
systematic  position  of,  163. 
Trimerelta,  243. 
Trimerocephalus,  153,  156. 
Trimerus,  154. 

glabella  and  pygidium  in,  155. 
Trinucleidse,  130,  131,  134. 
defined,  138. 

primitive  head  structure  in,  135. 
Trinudeus,  113,  116,  130,  138,  147,  177, 
178,  214,  220,  222. 
appendages  in,  223. 

in  pygidium  of,  135. 
concentricus,  221 . 

pygidium  of  young,  223. 
determination  of  sutures  in,  Bar- 
rande, 134. 
endopodites  of,  224. 
exopodites  of,  224. 
eye-line  in,  223. 
-spots  in,  135. 

in  young,  137. 
-tubercle  in,  221. 
fixed-cheeks  in,  127. 
free-cheeks  in,  124,  135. 
fringes  on  exopodites  of,  218. 
hypostoma  in,  GEhlert,  135. 
intestinal  canal  in,  209. 
lines  on  cephalon  of,  118. 
omatus,  177. 
pitted  border  in,  138. 
pygidium  in,  135. 


structure  and  appendages  of,  220. 

triangular  areas  in  adult,  222. 

in  young,  222. 
Triopus,  133. 
Triplecia,  245. 
Triplesia  putillus,  334. 
Tripocalpis  triserrata,  spines  in,  48. 
trocholophian  stage,  278. 
Trochurus,  150. 

Trophon  magellanicus,  spiniform  prom- 
inences on,  46. 
Tropidocoryphe,  148. 

eyes  in,  148. 

fixed-cheeks  in,  148. 
Tropidoleptus,  231,  246. 

zugolophus  stage  in,  281. 
Truncatulina  reticulata,  spines  in,  49. 
Tulotoma,  differentiation  in,  66. 
typembryo,  121,  247. 


Uncinulus,  246. 

Stricklandi,  314,  315. 
Uncites,  246. 
Ungulates,    development    of    horns  in 

horned,  47. 
Unio    spinosus,    anchoring    spines    on, 

Simpson,  46. 
Uralichas,  150. 
use  and  disuse,  law  of,  28. 
Uvigerina  aculeata,  spines  in,  49. 

asperula,  spinules  in,  49. 


valves  in  Brachiopoda,  difference    in, 

234. 
variation,  free,  35. 

among  Mollusca,  65. 

in    Achatinellae,    Hyatt    and 

Verrill,  36. 
law  of,  5. 
manner  of,  6. 
mechanical,  6. 
molar,  6. 
molecular,  6. 
physico-chemical,  6. 
progressive,  6. 


INDEX 


637 


regressive,  6. 
restrictions  of,  7. 
tendency  of,  6, 
variations  in  Achatinella  and  All&rchestes, 

36,  37. 

Vella,  spiniform  branches  of,  75. 
Verneuilina  spinulosa,  spines  in,  44. 
Vernon,  H.  M.,  reproductive  divergence, 

37. 

Verrill,  A.  E.,  Achatinellae,  36. 
cannibalistic  selection,  57. 
characters  in  appendages  of   Tri- 

nucleus,  225. 
importance  of  tabulas  in  a  natural 

classification  of  corals,  427. 
vertebrae,  spines  of,  9. 
vertebrates,  hornless  young  of  horned, 

19. 
Verworn,  development  of  extinct  Ostra- 

coda,  310. 
vitality,  ebbing,  Geddes,  12. 


W 


Waagen,  W.,  classification  of  Brachiop- 

oda,  242,  290. 

septal    and    dental    plates  in   the 

terebratuloids,  297. 

Wachsmuth,    C.,    and     Springer,    F., 

development  of  tubercles  of  Crinoidea 

and  Asteroidea  into  spines,  50. 

spines  on  ventral  sacs  of  Crinoidea, 

45. 

Walcott,  C.  D.,  additional  thoracic  seg- 
ments in  Trlarthrus  Becki,  217. 
biramous  appendages  in  Triarthrus 

200. 
casts  of  pedicles  of  fossil  Lingulae 

and  Eichwaldia,  257. 
classification  of  trilobites,  114. 
development  of  trilobites,  310. 
metamorphoses  of  trilobites,  166. 
mouth  in  Calymmene,  209. 
possible  trilobite  eggs,  169. 
young  of  Olenellus  asaphoides,  1 78. 
Waldheimia,4ll. 
bicarinata,  413. 
Mown,  413. 
recurved  loop  in,  413. 
septigera,  297. 


Wallace,  A.  R.,  on  the  origin  of  xeroph- 
ilous  plants,  74. 
protective  horns  in  males  of  Copri- 

dse  and  Dynastidse,  60. 
Whitfieldella,    absence    of    embryonal 
sinus  in,  388. 

nitida,  351,  352,  356,  373,  378,  392. 
discussion  of,  374. 

early  stages  of  growth  in,  373. 
tumida,  377. 
Whitfiddia,  377. 
Williams,  H.  S.,  variations  in  stock  of 

Atrypa  reticularis,  25. 
Woodward,  H.,  affinities  of  trilobites, 
110. 

Trilobita  the  earliest  forms  of  Crus- 
tacea, 184. 


Xanthidium     armatum,    spine     growth 

from  frustule  of,  43. 
Xiphogomium,  148. 
Xiphosphcera  polios,  spines  in,  48. 
Xylotrupes  gideon,  horns  in,  60. 


Youngia,  155,  156. 


Zacanthoides,  113,  142,  143,  144. 

pygidium  in,  143. 

Zaphrentis  spinulosa,  spinules  in,  50. 
Zellania,  246,  300,  304,  308. 

schizolophus  stage  in,  279. 
Zitta,  branches   of,    transformed    into 

spines,  74. 

Zittel,    K.    A.    von,    classification    of 
articulates,    112. 

of  Trilobita,  112,132. 
historical  review  of  trilobites,  111. 
oldest  species  of  Spondylus,  23. 
terminal  spine  in  Nasellarian  Radi- 

olaria,  43. 
zoe'a  of  common  crab,  56. 

of  Decapoda,  spines  in,  57. 
Zoic  maxima,  Gratacap,  35. 
zugolophus  stage,  281,  284. 


638 


INDEX 


Zygospira,  231,  246,  256,  270,  413,  415, 
416. 

brachial  supports  in,  415. 
brachidium  in,  284. 

Schuchert,  282. 
development  in,  411. 

of  brachial  supports  in,  410. 
minima,  315. 

modesta,  variation  in  brachial  sup- 
ports of,  416. 

whorls  in  spirals  of,  415. 
Nicoletti,  loop  in,  413,  416. 

rudimentary  spirals  in,  415. 
ontogeny  and  phylogeny  of,  417. 
pedicle-opening  in,  242. 


posterior  transverse  band  in,  416. 
primitive  arm  support  in,  411. 
recurvirostris,  416. 

brachidium  of,  284. 

development  of  brachial  sup- 
ports in,  413, 

lamellae  in  spirals  of,  415. 

loop  in,  413,  416. 

transverse  band  in,  413. 
resorption  of  loop  in,  283. 
Sqffbrdi,  loop  in,  413,  416. 

rudimentary  spirals  in,  415. 
to  Atrypa,  evolution  from,  417. 
trocholophus  stage  in,  283. 
whorls  to  a  cone  of,  415. 


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